Re: [PATCH v7 2/2] x86/tlb: skip redundant sync IPIs for native TLB flush

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

On 2026/3/23 19:10, David Hildenbrand (Arm) wrote:
> On 3/9/26 03:07, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> Enable the optimization introduced in the previous patch for x86.
> 
> Best to make the patch description standalone, not referring to
> "previous patch".

Good point. Will make the changelog standalone ;)

>>
>> 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.
>>
>> Two-step plan as David suggested[2]:
>>
>> 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/20260302145652.GH1395266@noisy.programming.kicks-ass.net/
>> [2] https://lore.kernel.org/linux-mm/bbfdf226-4660-4949-b17b-0d209ee4ef8c@kernel.org/
>>
>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
> 
> [...]
> 
>>   static inline void tlb_flush(struct mmu_gather *tlb)
>>   {
>>   	unsigned long start = 0UL, end = TLB_FLUSH_ALL;
>> @@ -20,7 +30,12 @@ static inline void tlb_flush(struct mmu_gather *tlb)
>>   		end = tlb->end;
>>   	}
>>   
>> -	flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables);
>> +	/*
>> +	 * Pass both freed_tables and unshared_tables so that lazy-TLB CPUs
>> +	 * also receive IPIs during unsharing page tables.
> 
> "unsharing of page tables" ?

Yes, that reads better.

> 
> I would maybe have it written ass
> 
> "Treat unshared_tables just like freed_tables, such that lazy-TLB CPUs
>   also receive IPIs during unsharing of page tables, allowing us to
>   safely implement tlb_table_flush_implies_ipi_broadcast()."
> 
>> +	 */
>> +	flush_tlb_mm_range(tlb->mm, start, end, stride_shift,
>> +			   tlb->freed_tables || tlb->unshared_tables);
>>   }

Cool, this wording is much clearer :)

> In general, LGTM.
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks for taking time to review!