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

[David Hildenbrand (Arm)]

On 3/9/26 03:07, Lance Yang wrote:
> From: Lance Yang <lance.yang@xxxxxxxxx>
> 
> Enable the optimization introduced in the previous patch for x86.

Best to make the patch description standalone, not referring to
"previous patch".

> 
> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/bbfdf226-4660-4949-b17b-0d209ee4ef8c@xxxxxxxxxx/
> 
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> ---

[...]

>  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" ?

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);
>  }

In general, LGTM.

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David