From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BB26E2BE058; Tue, 23 Dec 2025 11:13:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766488410; cv=none; b=BDdXSVNiIbcJ5uTjoQ9xRYFZa6jf21CLxBiOmGZegGRdPQs6zS4m+Jc/N+k5j8s7TRImVcoEnO8lE+iM9ao2wF/RWMAKhivqF1ugCv218XjUgNd158in0bfZGfXiVMP4iIcQgAiX9I3zl2RDR1Pu3u9hPP0BOPCOebQkoT90umk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766488410; c=relaxed/simple; bh=G7/C9/rO7ScpJXIbezmrJniAvV1BqDLKj7YzY62dxBc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jt5sD0dxKbVd6z1a7IKM7wcmRfX9i2haqBLTG1/uB/UMXlG80D+scJihbu64hQssVXszI862Nh72HLtulrtvrfhstyyI4MluVf1XoKmj427TsmQ0sDlgoQJQ0Wj5jMAInYBJyXkD+JEhPwLq7UR7fhzj8s1KUGerX+J9QUusBrQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=eJTxNHGq; arc=none smtp.client-ip=91.218.175.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="eJTxNHGq" Message-ID: <5071efdf-8260-43dc-8042-69414b124009@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1766488405; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ZhIVvsl+kpzgEGwl1JWdZa1JAIXGwZgZsmRbZEGTNGk=; b=eJTxNHGqd0zgRdGACW7l9uVB91p+hd7jUu8xLNxdY88PLcL/Ox23THWP7GIWdP9wPpRiVK IVda/s6snYL6blU82ENpfx/RRVXo65APAzlVgxSQYOdkwWDF8iWlAwFHkvCuIIomaJcSI4 dtkn2kjl8ImwtxZQD0hjSC52AQ7LQ94= Date: Tue, 23 Dec 2025 19:13:11 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH RFC 2/3] x86/mm: implement redundant IPI elimination for Content-Language: en-US To: "David Hildenbrand (Red Hat)" Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, aneesh.kumar@kernel.org, arnd@arndb.de, baohua@kernel.org, baolin.wang@linux.alibaba.com, bp@alien8.de, dave.hansen@linux.intel.com, dev.jain@arm.com, hpa@zytor.com, jannh@google.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mingo@redhat.com, npache@redhat.com, npiggin@gmail.com, peterz@infradead.org, riel@surriel.com, ryan.roberts@arm.com, shy828301@gmail.com, tglx@linutronix.de, will@kernel.org, x86@kernel.org, ziy@nvidia.com, Lance Yang References: <20251222031919.41964-1-ioworker0@gmail.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/12/23 17:44, David Hildenbrand (Red Hat) wrote: > On 12/22/25 04:19, Lance Yang wrote: >> From: Lance Yang >> >> >> On Thu, 18 Dec 2025 14:08:07 +0100, David Hildenbrand (Red Hat) wrote: >>> On 12/13/25 09:00, Lance Yang wrote: >>>> From: Lance Yang >>>> >>>> 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) >>>> Signed-off-by: Lance Yang >>>> --- >>>>    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 >>>>    #include >>>>    #include >>>>    #include >>>> +#include >>>> + >>>> +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. >> >> Cool! >> >>> >>> Something for x86 folks to give suggestions for. :) >> >> I prefer to use a boolean property instead of comparing function >> pointers. >> Something like this: >> >> ----8<---- >> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c >> index cfcb60468b01..90e9da33f2c7 100644 >> --- a/arch/x86/hyperv/mmu.c >> +++ b/arch/x86/hyperv/mmu.c >> @@ -243,4 +243,5 @@ void hyperv_setup_mmu_ops(void) >> >>       pr_info("Using hypercall for remote TLB flush\n"); >>       pv_ops.mmu.flush_tlb_multi = hyperv_flush_tlb_multi; >> +    pv_ops.mmu.tlb_flush_implies_ipi_broadcast = false; >>   } >> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/ >> asm/paravirt_types.h >> index 3502939415ad..f9756df6f3f6 100644 >> --- a/arch/x86/include/asm/paravirt_types.h >> +++ b/arch/x86/include/asm/paravirt_types.h >> @@ -133,6 +133,19 @@ struct pv_mmu_ops { >>       void (*flush_tlb_multi)(const struct cpumask *cpus, >>                   const struct flush_tlb_info *info); >> >> +    /* >> +     * Indicates whether TLB flush IPIs provide sufficient >> synchronization >> +     * for GUP-fast when freeing or unsharing page tables. >> +     * >> +     * Set to true only when the TLB flush guarantees: >> +     * - IPIs reach all CPUs with potentially stale paging-structure >> caches >> +     * - Synchronization with IRQ-disabled code like GUP-fast >> +     * >> +     * Paravirt implementations that use hypercalls (which may not send >> +     * real IPIs) should set this to false. >> +     */ >> +    bool tlb_flush_implies_ipi_broadcast; >> + >>       /* Hook for intercepting the destruction of an mm_struct. */ >>       void (*exit_mmap)(struct mm_struct *mm); >>       void (*notify_page_enc_status_changed)(unsigned long pfn, int >> npages, bool enc); >> diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h >> index 96602b7b7210..9d20ad4786cc 100644 >> --- a/arch/x86/include/asm/tlb.h >> +++ b/arch/x86/include/asm/tlb.h >> @@ -18,7 +18,7 @@ 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) >> +    if (!pv_ops.mmu.tlb_flush_implies_ipi_broadcast) >>           return false; >>   #endif >>       return !cpu_feature_enabled(X86_FEATURE_INVLPGB); > > I'd have thought that the X86_FEATURE_INVLPGB heck should then also be > taken care of by whoever sets tlb_flush_implies_ipi_broadcast. Makes sense! Let's have the INVLPGB check happen at setup time, not at use time :P Cheers, Lance