linux-kernel - Re: [PATCH RFC 2/3] x86/mm: implement redundant IPI elimination for PMD unsharing

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Message-ID: <c36501bc-2690-4ed2-b85e-13e64c41baaf@kernel.org>
Date: Thu, 18 Dec 2025 14:08:07 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org
Cc: will@...nel.org, aneesh.kumar@...nel.org, npiggin@...il.com,
 peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, arnd@...db.de,
 lorenzo.stoakes@...cle.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....com, baohua@...nel.org, ioworker0@...il.com,
 shy828301@...il.com, riel@...riel.com, jannh@...gle.com,
 linux-arch@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 2/3] x86/mm: implement redundant IPI elimination for
 PMD unsharing

On 12/13/25 09:00, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> Pass both freed_tables and unshared_tables to flush_tlb_mm_range() to
> ensure lazy-TLB CPUs receive IPIs and flush their paging-structure caches:
> 
> 	flush_tlb_mm_range(..., freed_tables || unshared_tables);
> 
> Implement tlb_table_flush_implies_ipi_broadcast() for x86: on native x86
> without paravirt or INVLPGB, the TLB flush IPI already provides necessary
> synchronization, allowing the second IPI to be skipped. For paravirt with
> non-native flush_tlb_multi and for INVLPGB, conservatively keep both IPIs.
> 
> Suggested-by: David Hildenbrand (Red Hat) <david@...nel.org>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>   arch/x86/include/asm/tlb.h | 17 ++++++++++++++++-
>   1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
> index 866ea78ba156..96602b7b7210 100644
> --- a/arch/x86/include/asm/tlb.h
> +++ b/arch/x86/include/asm/tlb.h
> @@ -5,10 +5,24 @@
>   #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
> +	/* Paravirt may use hypercalls that don't send real IPIs. */
> +	if (pv_ops.mmu.flush_tlb_multi != native_flush_tlb_multi)
> +		return false;
> +#endif
> +	return !cpu_feature_enabled(X86_FEATURE_INVLPGB);

Right, here I was wondering whether we should have a new pv_ops callback 
to indicate that instead.

pv_ops.mmu.tlb_table_flush_implies_ipi_broadcast()

Or a simple boolean property that pv init code properly sets.

Something for x86 folks to give suggestions for. :)

-- 
Cheers

David