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[2003:d8:2f34:c100:5d3c:50c0:398a:3ac9]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e2a996d7dsm136681855e9.4.2025.09.26.13.09.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Sep 2025 13:09:10 -0700 (PDT) Message-ID: <5d11b5f7-3208-4ea8-bbff-f535cf62d576@redhat.com> Date: Fri, 26 Sep 2025 22:09:07 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing To: Will Deacon , Patrick Roy Cc: Dave Hansen , "Roy, Patrick" , "pbonzini@redhat.com" , "corbet@lwn.net" , "maz@kernel.org" , "oliver.upton@linux.dev" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "luto@kernel.org" , "peterz@infradead.org" , "willy@infradead.org" , "akpm@linux-foundation.org" , "lorenzo.stoakes@oracle.com" , "Liam.Howlett@oracle.com" , "vbabka@suse.cz" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "song@kernel.org" , "jolsa@kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "yonghong.song@linux.dev" , "john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "shuah@kernel.org" , "seanjc@google.com" , "kvm@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "Cali, Marco" , "Kalyazin, Nikita" , "Thomson, Jack" , "derekmn@amazon.co.uk" , "tabba@google.com" , "ackerleytng@google.com" References: <20250924151101.2225820-4-patrick.roy@campus.lmu.de> <20250924152214.7292-1-roypat@amazon.co.uk> <20250924152214.7292-3-roypat@amazon.co.uk> <82bff1c4-987f-46cb-833c-bd99eaa46e7a@intel.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 26.09.25 12:53, Will Deacon wrote: > On Fri, Sep 26, 2025 at 10:46:15AM +0100, Patrick Roy wrote: >> >> >> On Thu, 2025-09-25 at 21:13 +0100, David Hildenbrand wrote: >>> On 25.09.25 21:59, Dave Hansen wrote: >>>> On 9/25/25 12:20, David Hildenbrand wrote: >>>>> On 25.09.25 20:27, Dave Hansen wrote: >>>>>> On 9/24/25 08:22, Roy, Patrick wrote: >>>>>>> Add an option to not perform TLB flushes after direct map manipulations. >>>>>> >>>>>> I'd really prefer this be left out for now. It's a massive can of worms. >>>>>> Let's agree on something that works and has well-defined behavior before >>>>>> we go breaking it on purpose. >>>>> >>>>> May I ask what the big concern here is? >>>> >>>> It's not a _big_ concern. >>> >>> Oh, I read "can of worms" and thought there is something seriously problematic :) >>> >>>> I just think we want to start on something >>>> like this as simple, secure, and deterministic as possible. >>> >>> Yes, I agree. And it should be the default. Less secure would have to be opt-in and documented thoroughly. >> >> Yes, I am definitely happy to have the 100% secure behavior be the >> default, and the skipping of TLB flushes be an opt-in, with thorough >> documentation! >> >> But I would like to include the "skip tlb flushes" option as part of >> this patch series straight away, because as I was alluding to in the >> commit message, with TLB flushes this is not usable for Firecracker for >> performance reasons :( > > I really don't want that option for arm64. If we're going to bother > unmapping from the linear map, we should invalidate the TLB. Reading "TLB flushes result in a up to 40x elongation of page faults in guest_memfd (scaling with the number of CPU cores), or a 5x elongation of memory population,", I can understand why one would want that optimization :) @Patrick, couldn't we use fallocate() to preallocate memory and batch the TLB flush within such an operation? That is, we wouldn't flush after each individual direct-map modification but after multiple ones part of a single operation like fallocate of a larger range. Likely wouldn't make all use cases happy. -- Cheers David / dhildenb