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b=JtKO4QANPZgLp/9I0GWBByVvq719GSIruX6T38eKeXJbZuk6oOGlRJ1N7/P4rO4Aj L8II8pmPgJYZ3/tJfC7RQG3g+SF6Wyxv0FY2PnunAYjKT9Id84VSQP51StTzOAMGLO nyaLpuARpVVwmphwmR6LWHr1FBOqgi3JEG83tXBjlwl15JrR0smgomY7DYIzzjHTv/ in6+KmWKw03h/FTwymyLjyw/Qw+lVs34o/qHpsIfVN7YQ1giMr/lS3VTJRo4Telj3+ /Eey6aTdgACkv1z1fUa++f4u+FHbKIwnnwIvWB1yqPaabd874pJUgp9sJe/g6TXml+ cqo/XBij2wgsw== Message-ID: Date: Mon, 23 Mar 2026 12:10:54 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 2/2] x86/tlb: skip redundant sync IPIs for native TLB flush To: Lance Yang , akpm@linux-foundation.org Cc: peterz@infradead.org, dave.hansen@intel.com, dave.hansen@linux.intel.com, ypodemsk@redhat.com, hughd@google.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, shy828301@gmail.com, riel@surriel.com, jannh@google.com, jgross@suse.com, seanjc@google.com, pbonzini@redhat.com, boris.ostrovsky@oracle.com, virtualization@lists.linux.dev, kvm@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ioworker0@gmail.com References: <20260309020711.20831-1-lance.yang@linux.dev> <20260309020711.20831-3-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: f4fdiomtruacrpnqojqhrnnyy9r5kqin X-Rspam-User: X-Rspamd-Queue-Id: 34A8318000B X-Rspamd-Server: rspam12 X-HE-Tag: 1774264265-678666 X-HE-Meta: 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/9/26 03:07, Lance Yang wrote: > From: Lance Yang > > 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@noisy.programming.kicks-ass.net/ > [2] https://lore.kernel.org/linux-mm/bbfdf226-4660-4949-b17b-0d209ee4ef8c@kernel.org/ > > Suggested-by: Peter Zijlstra > Suggested-by: David Hildenbrand (Arm) > Signed-off-by: Lance Yang > --- [...] > 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) -- Cheers, David