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

[Lance Yang <lance.yang@linux.dev>]

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