From: Lance Yang <[email protected]> Some page table operations need to synchronize with software/lockless walkers after a TLB flush by calling tlb_remove_table_sync_{one,rcu}(). On x86, that extra synchronization is redundant when the preceding TLB flush already broadcast IPIs to all relevant CPUs.
native_pv_tlb_init() checks whether native_flush_tlb_multi() is in use. On CONFIG_PARAVIRT systems, it checks pv_ops; on non-PARAVIRT, native flush is always in use. It decides once at boot whether to enable the optimization: if using native TLB flush and INVLPGB is not supported, we know IPIs were sent and can skip the redundant sync. The decision is fixed via a static key as Peter suggested[1]. PV backends (KVM, Xen, Hyper-V) typically have their own implementations and don't call native_flush_tlb_multi() directly, so they cannot be trusted to provide the IPI guarantees we need. Also rename the x86 flush_tlb_info bit from freed_tables to wake_lazy_cpus, as Dave suggested[2], to match the behavior it controls: whether the remote flush may skip CPUs in lazy TLB mode. Both freed_tables and unshared_tables set it, because lazy-TLB CPUs must receive IPIs before page tables can be freed or reused. With that guarantee in place, tlb_table_flush_implies_ipi_broadcast() can safely skip the later sync IPI. Two-step plan as David suggested[3]: Step 1 (this patch): Skip redundant sync when we're 100% certain the TLB flush sent IPIs. INVLPGB is excluded because when supported, we cannot guarantee IPIs were sent, keeping it clean and simple. Step 2 (future work): Send targeted IPIs only to CPUs actually doing software/lockless page table walks, benefiting all architectures. Regarding Step 2, it obviously only applies to setups where Step 1 does not apply: like x86 with INVLPGB or arm64. [1] https://lore.kernel.org/linux-mm/[email protected]/ [2] https://lore.kernel.org/linux-mm/[email protected]/ [3] https://lore.kernel.org/linux-mm/[email protected]/ Suggested-by: Dave Hansen <[email protected]> Suggested-by: Peter Zijlstra <[email protected]> Suggested-by: David Hildenbrand (Arm) <[email protected]> Signed-off-by: Lance Yang <[email protected]> --- arch/x86/hyperv/mmu.c | 4 ++-- arch/x86/include/asm/tlb.h | 19 +++++++++++++++- arch/x86/include/asm/tlbflush.h | 6 +++-- arch/x86/kernel/smpboot.c | 1 + arch/x86/mm/tlb.c | 39 +++++++++++++++++++++++---------- 5 files changed, 52 insertions(+), 17 deletions(-) diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c index cfcb60468b01..2cf1eeaffd6f 100644 --- a/arch/x86/hyperv/mmu.c +++ b/arch/x86/hyperv/mmu.c @@ -63,7 +63,7 @@ static void hyperv_flush_tlb_multi(const struct cpumask *cpus, struct hv_tlb_flush *flush; u64 status; unsigned long flags; - bool do_lazy = !info->freed_tables; + bool do_lazy = !info->wake_lazy_cpus; trace_hyperv_mmu_flush_tlb_multi(cpus, info); @@ -198,7 +198,7 @@ static u64 hyperv_flush_tlb_others_ex(const struct cpumask *cpus, flush->hv_vp_set.format = HV_GENERIC_SET_SPARSE_4K; nr_bank = cpumask_to_vpset_skip(&flush->hv_vp_set, cpus, - info->freed_tables ? NULL : cpu_is_lazy); + info->wake_lazy_cpus ? NULL : cpu_is_lazy); if (nr_bank < 0) return HV_STATUS_INVALID_PARAMETER; diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h index 866ea78ba156..fb256fd95f95 100644 --- a/arch/x86/include/asm/tlb.h +++ b/arch/x86/include/asm/tlb.h @@ -5,22 +5,39 @@ #define tlb_flush tlb_flush static inline void tlb_flush(struct mmu_gather *tlb); +#define tlb_table_flush_implies_ipi_broadcast tlb_table_flush_implies_ipi_broadcast +static inline bool tlb_table_flush_implies_ipi_broadcast(void); + #include <asm-generic/tlb.h> #include <linux/kernel.h> #include <vdso/bits.h> #include <vdso/page.h> +DECLARE_STATIC_KEY_FALSE(tlb_ipi_broadcast_key); + +static inline bool tlb_table_flush_implies_ipi_broadcast(void) +{ + return static_branch_likely(&tlb_ipi_broadcast_key); +} + static inline void tlb_flush(struct mmu_gather *tlb) { unsigned long start = 0UL, end = TLB_FLUSH_ALL; unsigned int stride_shift = tlb_get_unmap_shift(tlb); + /* + * Both freed_tables and unshared_tables must wake lazy-TLB CPUs, so + * they receive IPIs before reusing or freeing page tables, allowing + * us to safely implement tlb_table_flush_implies_ipi_broadcast(). + */ + bool wake_lazy_cpus = tlb->freed_tables || tlb->unshared_tables; + if (!tlb->fullmm && !tlb->need_flush_all) { start = tlb->start; end = tlb->end; } - flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables); + flush_tlb_mm_range(tlb->mm, start, end, stride_shift, wake_lazy_cpus); } static inline void invlpg(unsigned long addr) diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h index 5a3cdc439e38..39b9454781c3 100644 --- a/arch/x86/include/asm/tlbflush.h +++ b/arch/x86/include/asm/tlbflush.h @@ -18,6 +18,8 @@ DECLARE_PER_CPU(u64, tlbstate_untag_mask); +void __init native_pv_tlb_init(void); + void __flush_tlb_all(void); #define TLB_FLUSH_ALL -1UL @@ -225,7 +227,7 @@ struct flush_tlb_info { u64 new_tlb_gen; unsigned int initiating_cpu; u8 stride_shift; - u8 freed_tables; + u8 wake_lazy_cpus; u8 trim_cpumask; }; @@ -315,7 +317,7 @@ static inline bool mm_in_asid_transition(struct mm_struct *mm) { return false; } extern void flush_tlb_all(void); extern void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start, unsigned long end, unsigned int stride_shift, - bool freed_tables); + bool wake_lazy_cpus); extern void flush_tlb_kernel_range(unsigned long start, unsigned long end); static inline void flush_tlb_page(struct vm_area_struct *vma, unsigned long a) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 294a8ea60298..df776b645a9c 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1256,6 +1256,7 @@ void __init native_smp_prepare_boot_cpu(void) switch_gdt_and_percpu_base(me); native_pv_lock_init(); + native_pv_tlb_init(); } void __init native_smp_cpus_done(unsigned int max_cpus) diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 621e09d049cb..3ce254a3982c 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -26,6 +26,8 @@ #include "mm_internal.h" +DEFINE_STATIC_KEY_FALSE(tlb_ipi_broadcast_key); + #ifdef CONFIG_PARAVIRT # define STATIC_NOPV #else @@ -1360,16 +1362,16 @@ STATIC_NOPV void native_flush_tlb_multi(const struct cpumask *cpumask, (info->end - info->start) >> PAGE_SHIFT); /* - * If no page tables were freed, we can skip sending IPIs to - * CPUs in lazy TLB mode. They will flush the CPU themselves - * at the next context switch. + * If lazy-TLB CPUs do not need to be woken, we can skip sending + * IPIs to them. They will flush themselves at the next context + * switch. * - * However, if page tables are getting freed, we need to send the - * IPI everywhere, to prevent CPUs in lazy TLB mode from tripping - * up on the new contents of what used to be page tables, while - * doing a speculative memory access. + * However, if page tables are getting freed or unshared, we need + * to send the IPI everywhere, to prevent CPUs in lazy TLB mode + * from tripping up on the new contents of what used to be page + * tables, while doing a speculative memory access. */ - if (info->freed_tables || mm_in_asid_transition(info->mm)) + if (info->wake_lazy_cpus || mm_in_asid_transition(info->mm)) on_each_cpu_mask(cpumask, flush_tlb_func, (void *)info, true); else on_each_cpu_cond_mask(should_flush_tlb, flush_tlb_func, @@ -1402,7 +1404,7 @@ static DEFINE_PER_CPU(unsigned int, flush_tlb_info_idx); static struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm, unsigned long start, unsigned long end, - unsigned int stride_shift, bool freed_tables, + unsigned int stride_shift, bool wake_lazy_cpus, u64 new_tlb_gen) { struct flush_tlb_info *info = this_cpu_ptr(&flush_tlb_info); @@ -1429,7 +1431,7 @@ static struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm, info->end = end; info->mm = mm; info->stride_shift = stride_shift; - info->freed_tables = freed_tables; + info->wake_lazy_cpus = wake_lazy_cpus; info->new_tlb_gen = new_tlb_gen; info->initiating_cpu = smp_processor_id(); info->trim_cpumask = 0; @@ -1448,7 +1450,7 @@ static void put_flush_tlb_info(void) void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start, unsigned long end, unsigned int stride_shift, - bool freed_tables) + bool wake_lazy_cpus) { struct flush_tlb_info *info; int cpu = get_cpu(); @@ -1457,7 +1459,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start, /* This is also a barrier that synchronizes with switch_mm(). */ new_tlb_gen = inc_mm_tlb_gen(mm); - info = get_flush_tlb_info(mm, start, end, stride_shift, freed_tables, + info = get_flush_tlb_info(mm, start, end, stride_shift, wake_lazy_cpus, new_tlb_gen); /* @@ -1834,3 +1836,16 @@ static int __init create_tlb_single_page_flush_ceiling(void) return 0; } late_initcall(create_tlb_single_page_flush_ceiling); + +void __init native_pv_tlb_init(void) +{ +#ifdef CONFIG_PARAVIRT + if (pv_ops.mmu.flush_tlb_multi != native_flush_tlb_multi) + return; +#endif + + if (cpu_feature_enabled(X86_FEATURE_INVLPGB)) + return; + + static_branch_enable(&tlb_ipi_broadcast_key); +} -- 2.49.0
