Re: [PATCH v5 2/2] x86/tlb: skip redundant sync IPIs for native TLB flush

[Lance Yang]

On 2026/3/2 22:56, Peter Zijlstra wrote:
On Mon, Mar 02, 2026 at 02:30:36PM +0800, Lance Yang wrote:


@@ -221,3 +222,18 @@ NOKPROBE_SYMBOL(native_load_idt);
  
  EXPORT_SYMBOL(pv_ops);
  EXPORT_SYMBOL_GPL(pv_info);
+
+void __init native_pv_tlb_init(void)
+{
+       /*
+        * If PV backend already set the property, respect it.
+        * Otherwise, check if native TLB flush sends real IPIs to all target
+        * CPUs (i.e., not using INVLPGB broadcast invalidation).
+        */
+       if (pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast)
+               return;
+
+       if (pv_ops.mmu.flush_tlb_multi == native_flush_tlb_multi &&
+           !cpu_feature_enabled(X86_FEATURE_INVLPGB))
+               pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast = true;
+}

How about making this a static_branch instead?

Cool. Thanks for the suggestion!


diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
index 866ea78ba156..87ef7147eac8 100644
--- a/arch/x86/include/asm/tlb.h
+++ b/arch/x86/include/asm/tlb.h
@@ -5,10 +5,23 @@
  #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>
+#include <asm/paravirt.h>
+
+static inline bool tlb_table_flush_implies_ipi_broadcast(void)
+{
+#ifdef CONFIG_PARAVIRT
+       return pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast;
+#else
+       return !cpu_feature_enabled(X86_FEATURE_INVLPGB);
+#endif
+}

Then this turns into:

static inline bool tlb_table_flush_implies_ipi_broadcast(void)
{
        return static_branch_likely(&tlb_ipi_broadcast_key);
}

Right. That would be cleaner and faster, eliminating the branch overhead.

Trying using static_branch on top of this series, something like:

---8<---
diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
index 87ef7147eac8..409bbf335f26 100644
--- a/arch/x86/include/asm/tlb.h
+++ b/arch/x86/include/asm/tlb.h
@@ -10,17 +10,16 @@ static inline bool 
tlb_table_flush_implies_ipi_broadcast(void);

 #include <asm-generic/tlb.h>
 #include <linux/kernel.h>
+#include <linux/jump_label.h>
 #include <vdso/bits.h>
 #include <vdso/page.h>
 #include <asm/paravirt.h>

+DECLARE_STATIC_KEY_FALSE(tlb_ipi_broadcast_key);
+
 static inline bool tlb_table_flush_implies_ipi_broadcast(void)
 {
-#ifdef CONFIG_PARAVIRT
-       return pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast;
-#else
-       return !cpu_feature_enabled(X86_FEATURE_INVLPGB);
-#endif
+       return static_branch_likely(&tlb_ipi_broadcast_key);
 }

 static inline void tlb_flush(struct mmu_gather *tlb)
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index b681b8319295..bcf28980c319 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -15,6 +15,7 @@
 #include <linux/kprobes.h>
 #include <linux/pgtable.h>
 #include <linux/static_call.h>
+#include <linux/jump_label.h>

 #include <asm/bug.h>
 #include <asm/paravirt.h>
@@ -223,6 +224,8 @@ NOKPROBE_SYMBOL(native_load_idt);
 EXPORT_SYMBOL(pv_ops);
 EXPORT_SYMBOL_GPL(pv_info);

+DEFINE_STATIC_KEY_FALSE(tlb_ipi_broadcast_key);
+
 void __init native_pv_tlb_init(void)
 {
        /*
@@ -230,10 +233,14 @@ void __init native_pv_tlb_init(void)
         * Otherwise, check if native TLB flush sends real IPIs to all target
         * CPUs (i.e., not using INVLPGB broadcast invalidation).
         */
-       if (pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast)
+       if (pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast) {
+               static_branch_enable(&tlb_ipi_broadcast_key);
                return;
+       }

        if (pv_ops.mmu.flush_tlb_multi == native_flush_tlb_multi &&
-           !cpu_feature_enabled(X86_FEATURE_INVLPGB))
+           !cpu_feature_enabled(X86_FEATURE_INVLPGB)) {
                pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast = true;
+               static_branch_enable(&tlb_ipi_broadcast_key);
+       }
 }
---


Thanks,
Lance