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[2003:d8:2f3f:f800:c101:5c9f:3bc9:3d08]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fc82f2ff6sm4094888f8f.56.2025.09.25.13.13.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Sep 2025 13:13:50 -0700 (PDT) Message-ID: Date: Thu, 25 Sep 2025 22:13:47 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@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: Dave Hansen , "Roy, Patrick" Cc: "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" , "will@kernel.org" , "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 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. > > Let's say that with all the unmaps that load_unaligned_zeropad() faults > start to bite us. It'll take longer to find them if the TLB isn't flushed. > > Basically, it'll make the bad things happen sooner rather than later. Agreed. -- Cheers David / dhildenb