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[2003:d8:2f3f:f800:c101:5c9f:3bc9:3d08]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fc82f2ff6sm3956465f8f.56.2025.09.25.12.20.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Sep 2025 12:20:27 -0700 (PDT) Message-ID: Date: Thu, 25 Sep 2025 21:20:23 +0200 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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 77FBD160004 X-Stat-Signature: omrn6gcn3b7gddn3pcb3uaza4ss4u9oy X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1758828032-862379 X-HE-Meta: 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 yjzAMAYl 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? Not to challenge your position but to understand the involved problems and what would have to be documented at some point in a patch. Essentially we're removing the direct map from some memory we allocated through the buddy to reinstall it before we free the memory back to the buddy. So from the buddy POV whether we flush or don't flush the TLB shouldn't matter, right? Where the missing TLB flush would be relevant is to the workload (VM) where some (speculative) access through the direct map would be possible until the TLB was flushed. So until flushed, it's not-as-secure-as-you think. A flush after some time (batched over multiple page allocations?) could make it deterministic. Is there something else that's problematic? -- Cheers David / dhildenb