Re: [PATCH v7 09/12] x86/mm: enable broadcast TLB invalidation for multi-threaded processes

Manali Shukla
2025-01-30T05:39:29Z

On 1/23/2025 9:53 AM, Rik van Riel wrote:
> Use broadcast TLB invalidation, using the INVPLGB instruction, on AMD EPYC 3
> and newer CPUs.
>
> In order to not exhaust PCID space, and keep TLB flushes local for single
> threaded processes, we only hand out broadcast ASIDs to processes active on
> 3 or more CPUs, and gradually increase the threshold as broadcast ASID space
> is depleted.

Since the threshold is fixed at "3" and does not gradually increase in this
version, would it be more accurate to update the commit message to reflect this?

-Manali

>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> ---
> arch/x86/include/asm/mmu.h | 6 +
> arch/x86/include/asm/mmu_context.h | 14 ++
> arch/x86/include/asm/tlbflush.h | 73 ++++++
> arch/x86/mm/tlb.c | 344 ++++++++++++++++++++++++++++-
> 4 files changed, 425 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
> index 3b496cdcb74b..d71cd599fec4 100644
> --- a/arch/x86/include/asm/mmu.h
> +++ b/arch/x86/include/asm/mmu.h
> @@ -69,6 +69,12 @@ typedef struct {
> u16 pkey_allocation_map;
> s16 execute_only_pkey;
> #endif
> +
> +#ifdef CONFIG_X86_BROADCAST_TLB_FLUSH
> + u16 global_asid;
> + bool asid_transition;
> +#endif
> +
> } mm_context_t;
>
> #define INIT_MM_CONTEXT(mm) \
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 795fdd53bd0a..d670699d32c2 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -139,6 +139,8 @@ static inline void mm_reset_untag_mask(struct mm_struct *mm)
> #define enter_lazy_tlb enter_lazy_tlb
> extern void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk);
>
> +extern void destroy_context_free_global_asid(struct mm_struct *mm);
> +
> /*
> * Init a new mm. Used on mm copies, like at fork()
> * and on mm's that are brand-new, like at execve().
> @@ -161,6 +163,14 @@ static inline int init_new_context(struct task_struct *tsk,
> mm->context.execute_only_pkey = -1;
> }
> #endif
> +
> +#ifdef CONFIG_X86_BROADCAST_TLB_FLUSH
> + if (cpu_feature_enabled(X86_FEATURE_INVLPGB)) {
> + mm->context.global_asid = 0;
> + mm->context.asid_transition = false;
> + }
> +#endif
> +
> mm_reset_untag_mask(mm);
> init_new_context_ldt(mm);
> return 0;
> @@ -170,6 +180,10 @@ static inline int init_new_context(struct task_struct *tsk,
> static inline void destroy_context(struct mm_struct *mm)
> {
> destroy_context_ldt(mm);
> +#ifdef CONFIG_X86_BROADCAST_TLB_FLUSH
> + if (cpu_feature_enabled(X86_FEATURE_INVLPGB))
> + destroy_context_free_global_asid(mm);
> +#endif
> }
>
> extern void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index dba5caa4a9f4..7e2f3f7f6455 100644
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -6,6 +6,7 @@
> #include <linux/mmu_notifier.h>
> #include <linux/sched.h>
>
> +#include <asm/barrier.h>
> #include <asm/processor.h>
> #include <asm/cpufeature.h>
> #include <asm/special_insns.h>
> @@ -239,6 +240,78 @@ void flush_tlb_one_kernel(unsigned long addr);
> void flush_tlb_multi(const struct cpumask *cpumask,
> const struct flush_tlb_info *info);
>
> +#ifdef CONFIG_X86_BROADCAST_TLB_FLUSH
> +static inline bool is_dyn_asid(u16 asid)
> +{
> + if (!cpu_feature_enabled(X86_FEATURE_INVLPGB))
> + return true;
> +
> + return asid < TLB_NR_DYN_ASIDS;
> +}
> +
> +static inline bool is_global_asid(u16 asid)
> +{
> + return !is_dyn_asid(asid);
> +}
> +
> +static inline bool in_asid_transition(const struct flush_tlb_info *info)
> +{
> + if (!cpu_feature_enabled(X86_FEATURE_INVLPGB))
> + return false;
> +
> + return info->mm && READ_ONCE(info->mm->context.asid_transition);
> +}
> +
> +static inline u16 mm_global_asid(struct mm_struct *mm)
> +{
> + u16 asid;
> +
> + if (!cpu_feature_enabled(X86_FEATURE_INVLPGB))
> + return 0;
> +
> + asid = smp_load_acquire(&mm->context.global_asid);
> +
> + /* mm->context.global_asid is either 0, or a global ASID */
> + VM_WARN_ON_ONCE(asid && is_dyn_asid(asid));
> +
> + return asid;
> +}
> +#else
> +static inline bool is_dyn_asid(u16 asid)
> +{
> + return true;
> +}
> +
> +static inline bool is_global_asid(u16 asid)
> +{
> + return false;
> +}
> +
> +static inline bool in_asid_transition(const struct flush_tlb_info *info)
> +{
> + return false;
> +}
> +
> +static inline u16 mm_global_asid(struct mm_struct *mm)
> +{
> + return 0;
> +}
> +
> +static inline bool needs_global_asid_reload(struct mm_struct *next, u16 prev_asid)
> +{
> + return false;
> +}
> +
> +static inline void broadcast_tlb_flush(struct flush_tlb_info *info)
> +{
> + VM_WARN_ON_ONCE(1);
> +}
> +
> +static inline void consider_global_asid(struct mm_struct *mm)
> +{
> +}
> +#endif
> +
> #ifdef CONFIG_PARAVIRT
> #include <asm/paravirt.h>
> #endif
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 9d4864db5720..b55361fabb89 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -74,13 +74,15 @@
> * use different names for each of them:
> *
> * ASID - [0, TLB_NR_DYN_ASIDS-1]
> - * the canonical identifier for an mm
> + * the canonical identifier for an mm, dynamically allocated on each CPU
> + * [TLB_NR_DYN_ASIDS, MAX_ASID_AVAILABLE-1]
> + * the canonical, global identifier for an mm, identical across all CPUs
> *
> - * kPCID - [1, TLB_NR_DYN_ASIDS]
> + * kPCID - [1, MAX_ASID_AVAILABLE]
> * the value we write into the PCID part of CR3; corresponds to the
> * ASID+1, because PCID 0 is special.
> *
> - * uPCID - [2048 + 1, 2048 + TLB_NR_DYN_ASIDS]
> + * uPCID - [2048 + 1, 2048 + MAX_ASID_AVAILABLE]
> * for KPTI each mm has two address spaces and thus needs two
> * PCID values, but we can still do with a single ASID denomination
> * for each mm. Corresponds to kPCID + 2048.
> @@ -225,6 +227,20 @@ static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
> return;
> }
>
> + /*
> + * TLB consistency for global ASIDs is maintained with broadcast TLB
> + * flushing. The TLB is never outdated, and does not need flushing.
> + */
> + if (IS_ENABLED(CONFIG_X86_BROADCAST_TLB_FLUSH) && static_cpu_has(X86_FEATURE_INVLPGB)) {
> + u16 global_asid = mm_global_asid(next);
> +
> + if (global_asid) {
> + *new_asid = global_asid;
> + *need_flush = false;
> + return;
> + }
> + }
> +
> if (this_cpu_read(cpu_tlbstate.invalidate_other))
> clear_asid_other();
>
> @@ -251,6 +267,272 @@ static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
> *need_flush = true;
> }
>
> +#ifdef CONFIG_X86_BROADCAST_TLB_FLUSH
> +/*
> + * Logic for broadcast TLB invalidation.
> + */
> +static DEFINE_RAW_SPINLOCK(global_asid_lock);
> +static u16 last_global_asid = MAX_ASID_AVAILABLE;
> +static DECLARE_BITMAP(global_asid_used, MAX_ASID_AVAILABLE) = { 0 };
> +static DECLARE_BITMAP(global_asid_freed, MAX_ASID_AVAILABLE) = { 0 };
> +static int global_asid_available = MAX_ASID_AVAILABLE - TLB_NR_DYN_ASIDS - 1;
> +
> +static void reset_global_asid_space(void)
> +{
> + lockdep_assert_held(&global_asid_lock);
> +
> + /*
> + * A global TLB flush guarantees that any stale entries from
> + * previously freed global ASIDs get flushed from the TLB
> + * everywhere, making these global ASIDs safe to reuse.
> + */
> + invlpgb_flush_all_nonglobals();
> +
> + /*
> + * Clear all the previously freed global ASIDs from the
> + * broadcast_asid_used bitmap, now that the global TLB flush
> + * has made them actually available for re-use.
> + */
> + bitmap_andnot(global_asid_used, global_asid_used,
> + global_asid_freed, MAX_ASID_AVAILABLE);
> + bitmap_clear(global_asid_freed, 0, MAX_ASID_AVAILABLE);
> +
> + /*
> + * ASIDs 0-TLB_NR_DYN_ASIDS are used for CPU-local ASID
> + * assignments, for tasks doing IPI based TLB shootdowns.
> + * Restart the search from the start of the global ASID space.
> + */
> + last_global_asid = TLB_NR_DYN_ASIDS;
> +}
> +
> +static u16 get_global_asid(void)
> +{
> +
> + u16 asid;
> +
> + lockdep_assert_held(&global_asid_lock);
> +
> + /* The previous allocated ASID is at the top of the address space. */
> + if (last_global_asid >= MAX_ASID_AVAILABLE - 1)
> + reset_global_asid_space();
> +
> + asid = find_next_zero_bit(global_asid_used, MAX_ASID_AVAILABLE, last_global_asid);
> +
> + if (asid >= MAX_ASID_AVAILABLE) {
> + /* This should never happen. */
> + VM_WARN_ONCE(1, "Unable to allocate global ASID despite %d available\n", global_asid_available);
> + return 0;
> + }
> +
> + /* Claim this global ASID. */
> + __set_bit(asid, global_asid_used);
> + last_global_asid = asid;
> + global_asid_available--;
> + return asid;
> +}
> +
> +/*
> + * Returns true if the mm is transitioning from a CPU-local ASID to a global
> + * (INVLPGB) ASID, or the other way around.
> + */
> +static bool needs_global_asid_reload(struct mm_struct *next, u16 prev_asid)
> +{
> + u16 global_asid = mm_global_asid(next);
> +
> + if (global_asid && prev_asid != global_asid)
> + return true;
> +
> + if (!global_asid && is_global_asid(prev_asid))
> + return true;
> +
> + return false;
> +}
> +
> +void destroy_context_free_global_asid(struct mm_struct *mm)
> +{
> + if (!mm->context.global_asid)
> + return;
> +
> + guard(raw_spinlock_irqsave)(&global_asid_lock);
> +
> + /* The global ASID can be re-used only after flush at wrap-around. */
> + __set_bit(mm->context.global_asid, global_asid_freed);
> +
> + mm->context.global_asid = 0;
> + global_asid_available++;
> +}
> +
> +/*
> + * Check whether a process is currently active on more than "threshold" CPUs.
> + * This is a cheap estimation on whether or not it may make sense to assign
> + * a global ASID to this process, and use broadcast TLB invalidation.
> + */
> +static bool mm_active_cpus_exceeds(struct mm_struct *mm, int threshold)
> +{
> + int count = 0;
> + int cpu;
> +
> + /* This quick check should eliminate most single threaded programs. */
> + if (cpumask_weight(mm_cpumask(mm)) <= threshold)
> + return false;
> +
> + /* Slower check to make sure. */
> + for_each_cpu(cpu, mm_cpumask(mm)) {
> + /* Skip the CPUs that aren't really running this process. */
> + if (per_cpu(cpu_tlbstate.loaded_mm, cpu) != mm)
> + continue;
> +
> + if (per_cpu(cpu_tlbstate_shared.is_lazy, cpu))
> + continue;
> +
> + if (++count > threshold)
> + return true;
> + }
> + return false;
> +}
> +
> +/*
> + * Assign a global ASID to the current process, protecting against
> + * races between multiple threads in the process.
> + */
> +static void use_global_asid(struct mm_struct *mm)
> +{
> + u16 asid;
> +
> + guard(raw_spinlock_irqsave)(&global_asid_lock);
> +
> + /* This process is already using broadcast TLB invalidation. */
> + if (mm->context.global_asid)
> + return;
> +
> + /* The last global ASID was consumed while waiting for the lock. */
> + if (!global_asid_available) {
> + VM_WARN_ONCE(1, "Ran out of global ASIDs\n");
> + return;
> + }
> +
> + asid = get_global_asid();
> + if (!asid)
> + return;
> +
> + /*
> + * Notably flush_tlb_mm_range() -> broadcast_tlb_flush() ->
> + * finish_asid_transition() needs to observe asid_transition = true
> + * once it observes global_asid.
> + */
> + mm->context.asid_transition = true;
> + smp_store_release(&mm->context.global_asid, asid);
> +}
> +
> +static bool meets_global_asid_threshold(struct mm_struct *mm)
> +{
> + if (!global_asid_available)
> + return false;
> +
> + /*
> + * Assign a global ASID if the process is active on
> + * 4 or more CPUs simultaneously.
> + */
> + return mm_active_cpus_exceeds(mm, 3);
> +}
> +
> +static void consider_global_asid(struct mm_struct *mm)
> +{
> + if (!static_cpu_has(X86_FEATURE_INVLPGB))
> + return;
> +
> + /* Check every once in a while. */
> + if ((current->pid & 0x1f) != (jiffies & 0x1f))
> + return;
> +
> + if (meets_global_asid_threshold(mm))
> + use_global_asid(mm);
> +}
> +
> +static void finish_asid_transition(struct flush_tlb_info *info)
> +{
> + struct mm_struct *mm = info->mm;
> + int bc_asid = mm_global_asid(mm);
> + int cpu;
> +
> + if (!READ_ONCE(mm->context.asid_transition))
> + return;
> +
> + for_each_cpu(cpu, mm_cpumask(mm)) {
> + /*
> + * The remote CPU is context switching. Wait for that to
> + * finish, to catch the unlikely case of it switching to
> + * the target mm with an out of date ASID.
> + */
> + while (READ_ONCE(per_cpu(cpu_tlbstate.loaded_mm, cpu)) == LOADED_MM_SWITCHING)
> + cpu_relax();
> +
> + if (READ_ONCE(per_cpu(cpu_tlbstate.loaded_mm, cpu)) != mm)
> + continue;
> +
> + /*
> + * If at least one CPU is not using the global ASID yet,
> + * send a TLB flush IPI. The IPI should cause stragglers
> + * to transition soon.
> + *
> + * This can race with the CPU switching to another task;
> + * that results in a (harmless) extra IPI.
> + */
> + if (READ_ONCE(per_cpu(cpu_tlbstate.loaded_mm_asid, cpu)) != bc_asid) {
> + flush_tlb_multi(mm_cpumask(info->mm), info);
> + return;
> + }
> + }
> +
> + /* All the CPUs running this process are using the global ASID. */
> + WRITE_ONCE(mm->context.asid_transition, false);
> +}
> +
> +static void broadcast_tlb_flush(struct flush_tlb_info *info)
> +{
> + bool pmd = info->stride_shift == PMD_SHIFT;
> + unsigned long maxnr = invlpgb_count_max;
> + unsigned long asid = info->mm->context.global_asid;
> + unsigned long addr = info->start;
> + unsigned long nr;
> +
> + /* Flushing multiple pages at once is not supported with 1GB pages. */
> + if (info->stride_shift > PMD_SHIFT)
> + maxnr = 1;
> +
> + /*
> + * TLB flushes with INVLPGB are kicked off asynchronously.
> + * The inc_mm_tlb_gen() guarantees page table updates are done
> + * before these TLB flushes happen.
> + */
> + if (info->end == TLB_FLUSH_ALL) {
> + invlpgb_flush_single_pcid_nosync(kern_pcid(asid));
> + /* Do any CPUs supporting INVLPGB need PTI? */
> + if (static_cpu_has(X86_FEATURE_PTI))
> + invlpgb_flush_single_pcid_nosync(user_pcid(asid));
> + } else do {
> + /*
> + * Calculate how many pages can be flushed at once; if the
> + * remainder of the range is less than one page, flush one.
> + */
> + nr = min(maxnr, (info->end - addr) >> info->stride_shift);
> + nr = max(nr, 1);
> +
> + invlpgb_flush_user_nr_nosync(kern_pcid(asid), addr, nr, pmd);
> + /* Do any CPUs supporting INVLPGB need PTI? */
> + if (static_cpu_has(X86_FEATURE_PTI))
> + invlpgb_flush_user_nr_nosync(user_pcid(asid), addr, nr, pmd);
> +
> + addr += nr << info->stride_shift;
> + } while (addr < info->end);
> +
> + finish_asid_transition(info);
> +
> + /* Wait for the INVLPGBs kicked off above to finish. */
> + tlbsync();
> +}
> +#endif /* CONFIG_X86_BROADCAST_TLB_FLUSH */
> +
> /*
> * Given an ASID, flush the corresponding user ASID. We can delay this
> * until the next time we switch to it.
> @@ -556,8 +838,9 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
> */
> if (prev == next) {
> /* Not actually switching mm's */
> - VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> - next->context.ctx_id);
> + VM_WARN_ON(is_dyn_asid(prev_asid) &&
> + this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> + next->context.ctx_id);
>
> /*
> * If this races with another thread that enables lam, 'new_lam'
> @@ -573,6 +856,23 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
> !cpumask_test_cpu(cpu, mm_cpumask(next))))
> cpumask_set_cpu(cpu, mm_cpumask(next));
>
> + /*
> + * Check if the current mm is transitioning to a new ASID.
> + */
> + if (needs_global_asid_reload(next, prev_asid)) {
> + next_tlb_gen = atomic64_read(&next->context.tlb_gen);
> +
> + choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush);
> + goto reload_tlb;
> + }
> +
> + /*
> + * Broadcast TLB invalidation keeps this PCID up to date
> + * all the time.
> + */
> + if (is_global_asid(prev_asid))
> + return;
> +
> /*
> * If the CPU is not in lazy TLB mode, we are just switching
> * from one thread in a process to another thread in the same
> @@ -606,6 +906,13 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
> */
> cond_mitigation(tsk);
>
> + /*
> + * Let nmi_uaccess_okay() and finish_asid_transition()
> + * know that we're changing CR3.
> + */
> + this_cpu_write(cpu_tlbstate.loaded_mm, LOADED_MM_SWITCHING);
> + barrier();
> +
> /*
> * Leave this CPU in prev's mm_cpumask. Atomic writes to
> * mm_cpumask can be expensive under contention. The CPU
> @@ -620,14 +927,12 @@ void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *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();
> }
>
> +reload_tlb:
> new_lam = mm_lam_cr3_mask(next);
> if (need_flush) {
> + VM_WARN_ON_ONCE(is_global_asid(new_asid));
> 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, new_lam, true);
> @@ -746,7 +1051,7 @@ static void flush_tlb_func(void *info)
> const struct flush_tlb_info *f = info;
> 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 local_tlb_gen = this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen);
> + u64 local_tlb_gen;
> bool local = smp_processor_id() == f->initiating_cpu;
> unsigned long nr_invalidate = 0;
> u64 mm_tlb_gen;
> @@ -769,6 +1074,16 @@ static void flush_tlb_func(void *info)
> if (unlikely(loaded_mm == &init_mm))
> return;
>
> + /* Reload the ASID if transitioning into or out of a global ASID */
> + if (needs_global_asid_reload(loaded_mm, loaded_mm_asid)) {
> + switch_mm_irqs_off(NULL, loaded_mm, NULL);
> + loaded_mm_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
> + }
> +
> + /* Broadcast ASIDs are always kept up to date with INVLPGB. */
> + if (is_global_asid(loaded_mm_asid))
> + return;
> +
> VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].ctx_id) !=
> loaded_mm->context.ctx_id);
>
> @@ -786,6 +1101,8 @@ static void flush_tlb_func(void *info)
> return;
> }
>
> + local_tlb_gen = this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen);
> +
> if (unlikely(f->new_tlb_gen != TLB_GENERATION_INVALID &&
> f->new_tlb_gen <= local_tlb_gen)) {
> /*
> @@ -953,7 +1270,7 @@ STATIC_NOPV void native_flush_tlb_multi(const struct cpumask *cpumask,
> * up on the new contents of what used to be page tables, while
> * doing a speculative memory access.
> */
> - if (info->freed_tables)
> + if (info->freed_tables || in_asid_transition(info))
> on_each_cpu_mask(cpumask, flush_tlb_func, (void *)info, true);
> else
> on_each_cpu_cond_mask(should_flush_tlb, flush_tlb_func,
> @@ -1049,9 +1366,12 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
> * a local TLB flush is needed. Optimize this use-case by calling
> * flush_tlb_func_local() directly in this case.
> */
> - if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids) {
> + if (mm_global_asid(mm)) {
> + broadcast_tlb_flush(info);
> + } else if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids) {
> info->trim_cpumask = should_trim_cpumask(mm);
> flush_tlb_multi(mm_cpumask(mm), info);
> + consider_global_asid(mm);
> } else if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) {
> lockdep_assert_irqs_enabled();
> local_irq_disable();