GNU Linux-libre 4.19.264-gnu1

[releases.git] / arch / x86 / mm / tlb.c

#include <linux/init.h>

#include <linux/mm.h>
#include <linux/spinlock.h>
#include <linux/smp.h>
#include <linux/interrupt.h>
#include <linux/export.h>
#include <linux/cpu.h>
#include <linux/debugfs.h>

#include <asm/tlbflush.h>
#include <asm/mmu_context.h>
#include <asm/nospec-branch.h>
#include <asm/cache.h>
#include <asm/apic.h>
#include <asm/uv/uv.h>

/*
 *      TLB flushing, formerly SMP-only
 *              c/o Linus Torvalds.
 *
 *      These mean you can really definitely utterly forget about
 *      writing to user space from interrupts. (Its not allowed anyway).
 *
 *      Optimizations Manfred Spraul <manfred@colorfullife.com>
 *
 *      More scalable flush, from Andi Kleen
 *
 *      Implement flush IPI by CALL_FUNCTION_VECTOR, Alex Shi
 */

/*
 * Use bit 0 to mangle the TIF_SPEC_IB state into the mm pointer which is
 * stored in cpu_tlb_state.last_user_mm_ibpb.
 */
#define LAST_USER_MM_IBPB       0x1UL

/*
 * We get here when we do something requiring a TLB invalidation
 * but could not go invalidate all of the contexts.  We do the
 * necessary invalidation by clearing out the 'ctx_id' which
 * forces a TLB flush when the context is loaded.
 */
static void clear_asid_other(void)
{
        u16 asid;

        /*
         * This is only expected to be set if we have disabled
         * kernel _PAGE_GLOBAL pages.
         */
        if (!static_cpu_has(X86_FEATURE_PTI)) {
                WARN_ON_ONCE(1);
                return;
        }

        for (asid = 0; asid < TLB_NR_DYN_ASIDS; asid++) {
                /* Do not need to flush the current asid */
                if (asid == this_cpu_read(cpu_tlbstate.loaded_mm_asid))
                        continue;
                /*
                 * Make sure the next time we go to switch to
                 * this asid, we do a flush:
                 */
                this_cpu_write(cpu_tlbstate.ctxs[asid].ctx_id, 0);
        }
        this_cpu_write(cpu_tlbstate.invalidate_other, false);
}

atomic64_t last_mm_ctx_id = ATOMIC64_INIT(1);


static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
                            u16 *new_asid, bool *need_flush)
{
        u16 asid;

        if (!static_cpu_has(X86_FEATURE_PCID)) {
                *new_asid = 0;
                *need_flush = true;
                return;
        }

        if (this_cpu_read(cpu_tlbstate.invalidate_other))
                clear_asid_other();

        for (asid = 0; asid < TLB_NR_DYN_ASIDS; asid++) {
                if (this_cpu_read(cpu_tlbstate.ctxs[asid].ctx_id) !=
                    next->context.ctx_id)
                        continue;

                *new_asid = asid;
                *need_flush = (this_cpu_read(cpu_tlbstate.ctxs[asid].tlb_gen) <
                               next_tlb_gen);
                return;
        }

        /*
         * We don't currently own an ASID slot on this CPU.
         * Allocate a slot.
         */
        *new_asid = this_cpu_add_return(cpu_tlbstate.next_asid, 1) - 1;
        if (*new_asid >= TLB_NR_DYN_ASIDS) {
                *new_asid = 0;
                this_cpu_write(cpu_tlbstate.next_asid, 1);
        }
        *need_flush = true;
}

static void load_new_mm_cr3(pgd_t *pgdir, u16 new_asid, bool need_flush)
{
        unsigned long new_mm_cr3;

        if (need_flush) {
                invalidate_user_asid(new_asid);
                new_mm_cr3 = build_cr3(pgdir, new_asid);
        } else {
                new_mm_cr3 = build_cr3_noflush(pgdir, new_asid);
        }

        /*
         * Caution: many callers of this function expect
         * that load_cr3() is serializing and orders TLB
         * fills with respect to the mm_cpumask writes.
         */
        write_cr3(new_mm_cr3);
}

void leave_mm(int cpu)
{
        struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);

        /*
         * It's plausible that we're in lazy TLB mode while our mm is init_mm.
         * If so, our callers still expect us to flush the TLB, but there
         * aren't any user TLB entries in init_mm to worry about.
         *
         * This needs to happen before any other sanity checks due to
         * intel_idle's shenanigans.
         */
        if (loaded_mm == &init_mm)
                return;

        /* Warn if we're not lazy. */
        WARN_ON(!this_cpu_read(cpu_tlbstate.is_lazy));

        switch_mm(NULL, &init_mm, NULL);
}
EXPORT_SYMBOL_GPL(leave_mm);

void switch_mm(struct mm_struct *prev, struct mm_struct *next,
               struct task_struct *tsk)
{
        unsigned long flags;

        local_irq_save(flags);
        switch_mm_irqs_off(prev, next, tsk);
        local_irq_restore(flags);
}

static void sync_current_stack_to_mm(struct mm_struct *mm)
{
        unsigned long sp = current_stack_pointer;
        pgd_t *pgd = pgd_offset(mm, sp);

        if (pgtable_l5_enabled()) {
                if (unlikely(pgd_none(*pgd))) {
                        pgd_t *pgd_ref = pgd_offset_k(sp);

                        set_pgd(pgd, *pgd_ref);
                }
        } else {
                /*
                 * "pgd" is faked.  The top level entries are "p4d"s, so sync
                 * the p4d.  This compiles to approximately the same code as
                 * the 5-level case.
                 */
                p4d_t *p4d = p4d_offset(pgd, sp);

                if (unlikely(p4d_none(*p4d))) {
                        pgd_t *pgd_ref = pgd_offset_k(sp);
                        p4d_t *p4d_ref = p4d_offset(pgd_ref, sp);

                        set_p4d(p4d, *p4d_ref);
                }
        }
}

static inline unsigned long mm_mangle_tif_spec_ib(struct task_struct *next)
{
        unsigned long next_tif = task_thread_info(next)->flags;
        unsigned long ibpb = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_IBPB;

        return (unsigned long)next->mm | ibpb;
}

static void cond_ibpb(struct task_struct *next)
{
        if (!next || !next->mm)
                return;

        /*
         * Both, the conditional and the always IBPB mode use the mm
         * pointer to avoid the IBPB when switching between tasks of the
         * same process. Using the mm pointer instead of mm->context.ctx_id
         * opens a hypothetical hole vs. mm_struct reuse, which is more or
         * less impossible to control by an attacker. Aside of that it
         * would only affect the first schedule so the theoretically
         * exposed data is not really interesting.
         */
        if (static_branch_likely(&switch_mm_cond_ibpb)) {
                unsigned long prev_mm, next_mm;

                /*
                 * This is a bit more complex than the always mode because
                 * it has to handle two cases:
                 *
                 * 1) Switch from a user space task (potential attacker)
                 *    which has TIF_SPEC_IB set to a user space task
                 *    (potential victim) which has TIF_SPEC_IB not set.
                 *
                 * 2) Switch from a user space task (potential attacker)
                 *    which has TIF_SPEC_IB not set to a user space task
                 *    (potential victim) which has TIF_SPEC_IB set.
                 *
                 * This could be done by unconditionally issuing IBPB when
                 * a task which has TIF_SPEC_IB set is either scheduled in
                 * or out. Though that results in two flushes when:
                 *
                 * - the same user space task is scheduled out and later
                 *   scheduled in again and only a kernel thread ran in
                 *   between.
                 *
                 * - a user space task belonging to the same process is
                 *   scheduled in after a kernel thread ran in between
                 *
                 * - a user space task belonging to the same process is
                 *   scheduled in immediately.
                 *
                 * Optimize this with reasonably small overhead for the
                 * above cases. Mangle the TIF_SPEC_IB bit into the mm
                 * pointer of the incoming task which is stored in
                 * cpu_tlbstate.last_user_mm_ibpb for comparison.
                 */
                next_mm = mm_mangle_tif_spec_ib(next);
                prev_mm = this_cpu_read(cpu_tlbstate.last_user_mm_ibpb);

                /*
                 * Issue IBPB only if the mm's are different and one or
                 * both have the IBPB bit set.
                 */
                if (next_mm != prev_mm &&
                    (next_mm | prev_mm) & LAST_USER_MM_IBPB)
                        indirect_branch_prediction_barrier();

                this_cpu_write(cpu_tlbstate.last_user_mm_ibpb, next_mm);
        }

        if (static_branch_unlikely(&switch_mm_always_ibpb)) {
                /*
                 * Only flush when switching to a user space task with a
                 * different context than the user space task which ran
                 * last on this CPU.
                 */
                if (this_cpu_read(cpu_tlbstate.last_user_mm) != next->mm) {
                        indirect_branch_prediction_barrier();
                        this_cpu_write(cpu_tlbstate.last_user_mm, next->mm);
                }
        }
}

void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
                        struct task_struct *tsk)
{
        struct mm_struct *real_prev = this_cpu_read(cpu_tlbstate.loaded_mm);
        u16 prev_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
        unsigned cpu = smp_processor_id();
        u64 next_tlb_gen;

        /*
         * NB: The scheduler will call us with prev == next when switching
         * from lazy TLB mode to normal mode if active_mm isn't changing.
         * When this happens, we don't assume that CR3 (and hence
         * cpu_tlbstate.loaded_mm) matches next.
         *
         * NB: leave_mm() calls us with prev == NULL and tsk == NULL.
         */

        /* We don't want flush_tlb_func_* to run concurrently with us. */
        if (IS_ENABLED(CONFIG_PROVE_LOCKING))
                WARN_ON_ONCE(!irqs_disabled());

        /*
         * Verify that CR3 is what we think it is.  This will catch
         * hypothetical buggy code that directly switches to swapper_pg_dir
         * without going through leave_mm() / switch_mm_irqs_off() or that
         * does something like write_cr3(read_cr3_pa()).
         *
         * Only do this check if CONFIG_DEBUG_VM=y because __read_cr3()
         * isn't free.
         */
#ifdef CONFIG_DEBUG_VM
        if (WARN_ON_ONCE(__read_cr3() != build_cr3(real_prev->pgd, prev_asid))) {
                /*
                 * If we were to BUG here, we'd be very likely to kill
                 * the system so hard that we don't see the call trace.
                 * Try to recover instead by ignoring the error and doing
                 * a global flush to minimize the chance of corruption.
                 *
                 * (This is far from being a fully correct recovery.
                 *  Architecturally, the CPU could prefetch something
                 *  back into an incorrect ASID slot and leave it there
                 *  to cause trouble down the road.  It's better than
                 *  nothing, though.)
                 */
                __flush_tlb_all();
        }
#endif
        this_cpu_write(cpu_tlbstate.is_lazy, false);

        /*
         * The membarrier system call requires a full memory barrier and
         * core serialization before returning to user-space, after
         * storing to rq->curr, when changing mm.  This is because
         * membarrier() sends IPIs to all CPUs that are in the target mm
         * to make them issue memory barriers.  However, if another CPU
         * switches to/from the target mm concurrently with
         * membarrier(), it can cause that CPU not to receive an IPI
         * when it really should issue a memory barrier.  Writing to CR3
         * provides that full memory barrier and core serializing
         * instruction.
         */
        if (real_prev == next) {
                VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
                           next->context.ctx_id);

                /*
                 * We don't currently support having a real mm loaded without
                 * our cpu set in mm_cpumask().  We have all the bookkeeping
                 * in place to figure out whether we would need to flush
                 * if our cpu were cleared in mm_cpumask(), but we don't
                 * currently use it.
                 */
                if (WARN_ON_ONCE(real_prev != &init_mm &&
                                 !cpumask_test_cpu(cpu, mm_cpumask(next))))
                        cpumask_set_cpu(cpu, mm_cpumask(next));

                return;
        } else {
                u16 new_asid;
                bool need_flush;

                /*
                 * Avoid user/user BTB poisoning by flushing the branch
                 * predictor when switching between processes. This stops
                 * one process from doing Spectre-v2 attacks on another.
                 */
                cond_ibpb(tsk);

                if (IS_ENABLED(CONFIG_VMAP_STACK)) {
                        /*
                         * If our current stack is in vmalloc space and isn't
                         * mapped in the new pgd, we'll double-fault.  Forcibly
                         * map it.
                         */
                        sync_current_stack_to_mm(next);
                }

                /*
                 * Stop remote flushes for the previous mm.
                 * Skip kernel threads; we never send init_mm TLB flushing IPIs,
                 * but the bitmap manipulation can cause cache line contention.
                 */
                if (real_prev != &init_mm) {
                        VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu,
                                                mm_cpumask(real_prev)));
                        cpumask_clear_cpu(cpu, mm_cpumask(real_prev));
                }

                /*
                 * Start remote flushes and then read tlb_gen.
                 */
                if (next != &init_mm)
                        cpumask_set_cpu(cpu, mm_cpumask(next));
                next_tlb_gen = atomic64_read(&next->context.tlb_gen);

                choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush);

                /* Let nmi_uaccess_okay() know that we're changing CR3. */
                this_cpu_write(cpu_tlbstate.loaded_mm, LOADED_MM_SWITCHING);
                barrier();

                if (need_flush) {
                        this_cpu_write(cpu_tlbstate.ctxs[new_asid].ctx_id, next->context.ctx_id);
                        this_cpu_write(cpu_tlbstate.ctxs[new_asid].tlb_gen, next_tlb_gen);
                        load_new_mm_cr3(next->pgd, new_asid, true);

                        /*
                         * NB: This gets called via leave_mm() in the idle path
                         * where RCU functions differently.  Tracing normally
                         * uses RCU, so we need to use the _rcuidle variant.
                         *
                         * (There is no good reason for this.  The idle code should
                         *  be rearranged to call this before rcu_idle_enter().)
                         */
                        trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
                } else {
                        /* The new ASID is already up to date. */
                        load_new_mm_cr3(next->pgd, new_asid, false);

                        /* See above wrt _rcuidle. */
                        trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, 0);
                }

                /* Make sure we write CR3 before loaded_mm. */
                barrier();

                this_cpu_write(cpu_tlbstate.loaded_mm, next);
                this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);
        }

        load_mm_cr4(next);
        switch_ldt(real_prev, next);
}

/*
 * Please ignore the name of this function.  It should be called
 * switch_to_kernel_thread().
 *
 * enter_lazy_tlb() is a hint from the scheduler that we are entering a
 * kernel thread or other context without an mm.  Acceptable implementations
 * include doing nothing whatsoever, switching to init_mm, or various clever
 * lazy tricks to try to minimize TLB flushes.
 *
 * The scheduler reserves the right to call enter_lazy_tlb() several times
 * in a row.  It will notify us that we're going back to a real mm by
 * calling switch_mm_irqs_off().
 */
void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
{
        if (this_cpu_read(cpu_tlbstate.loaded_mm) == &init_mm)
                return;

        if (tlb_defer_switch_to_init_mm()) {
                /*
                 * There's a significant optimization that may be possible
                 * here.  We have accurate enough TLB flush tracking that we
                 * don't need to maintain coherence of TLB per se when we're
                 * lazy.  We do, however, need to maintain coherence of
                 * paging-structure caches.  We could, in principle, leave our
                 * old mm loaded and only switch to init_mm when
                 * tlb_remove_page() happens.
                 */
                this_cpu_write(cpu_tlbstate.is_lazy, true);
        } else {
                switch_mm(NULL, &init_mm, NULL);
        }
}

/*
 * Call this when reinitializing a CPU.  It fixes the following potential
 * problems:
 *
 * - The ASID changed from what cpu_tlbstate thinks it is (most likely
 *   because the CPU was taken down and came back up with CR3's PCID
 *   bits clear.  CPU hotplug can do this.
 *
 * - The TLB contains junk in slots corresponding to inactive ASIDs.
 *
 * - The CPU went so far out to lunch that it may have missed a TLB
 *   flush.
 */
void initialize_tlbstate_and_flush(void)
{
        int i;
        struct mm_struct *mm = this_cpu_read(cpu_tlbstate.loaded_mm);
        u64 tlb_gen = atomic64_read(&init_mm.context.tlb_gen);
        unsigned long cr3 = __read_cr3();

        /* Assert that CR3 already references the right mm. */
        WARN_ON((cr3 & CR3_ADDR_MASK) != __pa(mm->pgd));

        /*
         * Assert that CR4.PCIDE is set if needed.  (CR4.PCIDE initialization
         * doesn't work like other CR4 bits because it can only be set from
         * long mode.)
         */
        WARN_ON(boot_cpu_has(X86_FEATURE_PCID) &&
                !(cr4_read_shadow() & X86_CR4_PCIDE));

        /* Force ASID 0 and force a TLB flush. */
        write_cr3(build_cr3(mm->pgd, 0));

        /* Reinitialize tlbstate. */
        this_cpu_write(cpu_tlbstate.last_user_mm_ibpb, LAST_USER_MM_IBPB);
        this_cpu_write(cpu_tlbstate.loaded_mm_asid, 0);
        this_cpu_write(cpu_tlbstate.next_asid, 1);
        this_cpu_write(cpu_tlbstate.ctxs[0].ctx_id, mm->context.ctx_id);
        this_cpu_write(cpu_tlbstate.ctxs[0].tlb_gen, tlb_gen);

        for (i = 1; i < TLB_NR_DYN_ASIDS; i++)
                this_cpu_write(cpu_tlbstate.ctxs[i].ctx_id, 0);
}

/*
 * flush_tlb_func_common()'s memory ordering requirement is that any
 * TLB fills that happen after we flush the TLB are ordered after we
 * read active_mm's tlb_gen.  We don't need any explicit barriers
 * because all x86 flush operations are serializing and the
 * atomic64_read operation won't be reordered by the compiler.
 */
static void flush_tlb_func_common(const struct flush_tlb_info *f,
                                  bool local, enum tlb_flush_reason reason)
{
        /*
         * We have three different tlb_gen values in here.  They are:
         *
         * - mm_tlb_gen:     the latest generation.
         * - local_tlb_gen:  the generation that this CPU has already caught
         *                   up to.
         * - f->new_tlb_gen: the generation that the requester of the flush
         *                   wants us to catch up to.
         */
        struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
        u32 loaded_mm_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
        u64 mm_tlb_gen = atomic64_read(&loaded_mm->context.tlb_gen);
        u64 local_tlb_gen = this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen);

        /* This code cannot presently handle being reentered. */
        VM_WARN_ON(!irqs_disabled());

        if (unlikely(loaded_mm == &init_mm))
                return;

        VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].ctx_id) !=
                   loaded_mm->context.ctx_id);

        if (this_cpu_read(cpu_tlbstate.is_lazy)) {
                /*
                 * We're in lazy mode.  We need to at least flush our
                 * paging-structure cache to avoid speculatively reading
                 * garbage into our TLB.  Since switching to init_mm is barely
                 * slower than a minimal flush, just switch to init_mm.
                 */
                switch_mm_irqs_off(NULL, &init_mm, NULL);
                return;
        }

        if (unlikely(local_tlb_gen == mm_tlb_gen)) {
                /*
                 * There's nothing to do: we're already up to date.  This can
                 * happen if two concurrent flushes happen -- the first flush to
                 * be handled can catch us all the way up, leaving no work for
                 * the second flush.
                 */
                trace_tlb_flush(reason, 0);
                return;
        }

        WARN_ON_ONCE(local_tlb_gen > mm_tlb_gen);
        WARN_ON_ONCE(f->new_tlb_gen > mm_tlb_gen);

        /*
         * If we get to this point, we know that our TLB is out of date.
         * This does not strictly imply that we need to flush (it's
         * possible that f->new_tlb_gen <= local_tlb_gen), but we're
         * going to need to flush in the very near future, so we might
         * as well get it over with.
         *
         * The only question is whether to do a full or partial flush.
         *
         * We do a partial flush if requested and two extra conditions
         * are met:
         *
         * 1. f->new_tlb_gen == local_tlb_gen + 1.  We have an invariant that
         *    we've always done all needed flushes to catch up to
         *    local_tlb_gen.  If, for example, local_tlb_gen == 2 and
         *    f->new_tlb_gen == 3, then we know that the flush needed to bring
         *    us up to date for tlb_gen 3 is the partial flush we're
         *    processing.
         *
         *    As an example of why this check is needed, suppose that there
         *    are two concurrent flushes.  The first is a full flush that
         *    changes context.tlb_gen from 1 to 2.  The second is a partial
         *    flush that changes context.tlb_gen from 2 to 3.  If they get
         *    processed on this CPU in reverse order, we'll see
         *     local_tlb_gen == 1, mm_tlb_gen == 3, and end != TLB_FLUSH_ALL.
         *    If we were to use __flush_tlb_one_user() and set local_tlb_gen to
         *    3, we'd be break the invariant: we'd update local_tlb_gen above
         *    1 without the full flush that's needed for tlb_gen 2.
         *
         * 2. f->new_tlb_gen == mm_tlb_gen.  This is purely an optimiation.
         *    Partial TLB flushes are not all that much cheaper than full TLB
         *    flushes, so it seems unlikely that it would be a performance win
         *    to do a partial flush if that won't bring our TLB fully up to
         *    date.  By doing a full flush instead, we can increase
         *    local_tlb_gen all the way to mm_tlb_gen and we can probably
         *    avoid another flush in the very near future.
         */
        if (f->end != TLB_FLUSH_ALL &&
            f->new_tlb_gen == local_tlb_gen + 1 &&
            f->new_tlb_gen == mm_tlb_gen) {
                /* Partial flush */
                unsigned long addr;
                unsigned long nr_pages = (f->end - f->start) >> PAGE_SHIFT;

                addr = f->start;
                while (addr < f->end) {
                        __flush_tlb_one_user(addr);
                        addr += PAGE_SIZE;
                }
                if (local)
                        count_vm_tlb_events(NR_TLB_LOCAL_FLUSH_ONE, nr_pages);
                trace_tlb_flush(reason, nr_pages);
        } else {
                /* Full flush. */
                local_flush_tlb();
                if (local)
                        count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ALL);
                trace_tlb_flush(reason, TLB_FLUSH_ALL);
        }

        /* Both paths above update our state to mm_tlb_gen. */
        this_cpu_write(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen, mm_tlb_gen);
}

static void flush_tlb_func_local(void *info, enum tlb_flush_reason reason)
{
        const struct flush_tlb_info *f = info;

        flush_tlb_func_common(f, true, reason);
}

static void flush_tlb_func_remote(void *info)
{
        const struct flush_tlb_info *f = info;

        inc_irq_stat(irq_tlb_count);

        if (f->mm && f->mm != this_cpu_read(cpu_tlbstate.loaded_mm))
                return;

        count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED);
        flush_tlb_func_common(f, false, TLB_REMOTE_SHOOTDOWN);
}

void native_flush_tlb_others(const struct cpumask *cpumask,
                             const struct flush_tlb_info *info)
{
        count_vm_tlb_event(NR_TLB_REMOTE_FLUSH);
        if (info->end == TLB_FLUSH_ALL)
                trace_tlb_flush(TLB_REMOTE_SEND_IPI, TLB_FLUSH_ALL);
        else
                trace_tlb_flush(TLB_REMOTE_SEND_IPI,
                                (info->end - info->start) >> PAGE_SHIFT);

        if (is_uv_system()) {
                /*
                 * This whole special case is confused.  UV has a "Broadcast
                 * Assist Unit", which seems to be a fancy way to send IPIs.
                 * Back when x86 used an explicit TLB flush IPI, UV was
                 * optimized to use its own mechanism.  These days, x86 uses
                 * smp_call_function_many(), but UV still uses a manual IPI,
                 * and that IPI's action is out of date -- it does a manual
                 * flush instead of calling flush_tlb_func_remote().  This
                 * means that the percpu tlb_gen variables won't be updated
                 * and we'll do pointless flushes on future context switches.
                 *
                 * Rather than hooking native_flush_tlb_others() here, I think
                 * that UV should be updated so that smp_call_function_many(),
                 * etc, are optimal on UV.
                 */
                cpumask = uv_flush_tlb_others(cpumask, info);
                if (cpumask)
                        smp_call_function_many(cpumask, flush_tlb_func_remote,
                                               (void *)info, 1);
                return;
        }
        smp_call_function_many(cpumask, flush_tlb_func_remote,
                               (void *)info, 1);
}

/*
 * See Documentation/x86/tlb.txt for details.  We choose 33
 * because it is large enough to cover the vast majority (at
 * least 95%) of allocations, and is small enough that we are
 * confident it will not cause too much overhead.  Each single
 * flush is about 100 ns, so this caps the maximum overhead at
 * _about_ 3,000 ns.
 *
 * This is in units of pages.
 */
static unsigned long tlb_single_page_flush_ceiling __read_mostly = 33;

void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
                                unsigned long end, unsigned long vmflag)
{
        int cpu;

        struct flush_tlb_info info = {
                .mm = mm,
        };

        cpu = get_cpu();

        /* This is also a barrier that synchronizes with switch_mm(). */
        info.new_tlb_gen = inc_mm_tlb_gen(mm);

        /* Should we flush just the requested range? */
        if ((end != TLB_FLUSH_ALL) &&
            !(vmflag & VM_HUGETLB) &&
            ((end - start) >> PAGE_SHIFT) <= tlb_single_page_flush_ceiling) {
                info.start = start;
                info.end = end;
        } else {
                info.start = 0UL;
                info.end = TLB_FLUSH_ALL;
        }

        if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) {
                VM_WARN_ON(irqs_disabled());
                local_irq_disable();
                flush_tlb_func_local(&info, TLB_LOCAL_MM_SHOOTDOWN);
                local_irq_enable();
        }

        if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
                flush_tlb_others(mm_cpumask(mm), &info);

        put_cpu();
}


static void do_flush_tlb_all(void *info)
{
        count_vm_tlb_event(NR_TLB_REMOTE_FLUSH_RECEIVED);
        __flush_tlb_all();
}

void flush_tlb_all(void)
{
        count_vm_tlb_event(NR_TLB_REMOTE_FLUSH);
        on_each_cpu(do_flush_tlb_all, NULL, 1);
}

static void do_kernel_range_flush(void *info)
{
        struct flush_tlb_info *f = info;
        unsigned long addr;

        /* flush range by one by one 'invlpg' */
        for (addr = f->start; addr < f->end; addr += PAGE_SIZE)
                __flush_tlb_one_kernel(addr);
}

void flush_tlb_kernel_range(unsigned long start, unsigned long end)
{

        /* Balance as user space task's flush, a bit conservative */
        if (end == TLB_FLUSH_ALL ||
            (end - start) > tlb_single_page_flush_ceiling << PAGE_SHIFT) {
                on_each_cpu(do_flush_tlb_all, NULL, 1);
        } else {
                struct flush_tlb_info info;
                info.start = start;
                info.end = end;
                on_each_cpu(do_kernel_range_flush, &info, 1);
        }
}

void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
{
        struct flush_tlb_info info = {
                .mm = NULL,
                .start = 0UL,
                .end = TLB_FLUSH_ALL,
        };

        int cpu = get_cpu();

        if (cpumask_test_cpu(cpu, &batch->cpumask)) {
                VM_WARN_ON(irqs_disabled());
                local_irq_disable();
                flush_tlb_func_local(&info, TLB_LOCAL_SHOOTDOWN);
                local_irq_enable();
        }

        if (cpumask_any_but(&batch->cpumask, cpu) < nr_cpu_ids)
                flush_tlb_others(&batch->cpumask, &info);

        cpumask_clear(&batch->cpumask);

        put_cpu();
}

static ssize_t tlbflush_read_file(struct file *file, char __user *user_buf,
                             size_t count, loff_t *ppos)
{
        char buf[32];
        unsigned int len;

        len = sprintf(buf, "%ld\n", tlb_single_page_flush_ceiling);
        return simple_read_from_buffer(user_buf, count, ppos, buf, len);
}

static ssize_t tlbflush_write_file(struct file *file,
                 const char __user *user_buf, size_t count, loff_t *ppos)
{
        char buf[32];
        ssize_t len;
        int ceiling;

        len = min(count, sizeof(buf) - 1);
        if (copy_from_user(buf, user_buf, len))
                return -EFAULT;

        buf[len] = '\0';
        if (kstrtoint(buf, 0, &ceiling))
                return -EINVAL;

        if (ceiling < 0)
                return -EINVAL;

        tlb_single_page_flush_ceiling = ceiling;
        return count;
}

static const struct file_operations fops_tlbflush = {
        .read = tlbflush_read_file,
        .write = tlbflush_write_file,
        .llseek = default_llseek,
};

static int __init create_tlb_single_page_flush_ceiling(void)
{
        debugfs_create_file("tlb_single_page_flush_ceiling", S_IRUSR | S_IWUSR,
                            arch_debugfs_dir, NULL, &fops_tlbflush);
        return 0;
}
late_initcall(create_tlb_single_page_flush_ceiling);