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

[Lance Yang]

On 2026/3/23 19:10, David Hildenbrand (Arm) wrote:
> 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".

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@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" ?

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@xxxxxxxxxx>

Thanks for taking time to review!