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[213.44.135.146]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-426f2f72e18sm1737515f8f.0.2025.10.14.04.02.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Oct 2025 04:02:52 -0700 (PDT) From: Valentin Schneider To: Sean Christopherson Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, linux-arch@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Josh Poimboeuf , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Arnaldo Carvalho de Melo , Paolo Bonzini , Arnd Bergmann , Frederic Weisbecker , "Paul E. McKenney" , Jason Baron , Steven Rostedt , Ard Biesheuvel , Sami Tolvanen , "David S. Miller" , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Mel Gorman , Andrew Morton , Masahiro Yamada , Han Shen , Rik van Riel , Jann Horn , Dan Carpenter , Oleg Nesterov , Juri Lelli , Clark Williams , Yair Podemsky , Marcelo Tosatti , Daniel Wagner , Petr Tesarik Subject: Re: [PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush_cond keys as allowed in .noinstr In-Reply-To: References: <20251010153839.151763-1-vschneid@redhat.com> <20251010153839.151763-20-vschneid@redhat.com> Date: Tue, 14 Oct 2025 13:02:50 +0200 Message-ID: MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: GaIRNbrcQd9BJo9byqE2XKKZdfKLf1TXgc_5vZEErF8_1760439775 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-Rspam-User: X-Rspamd-Queue-Id: DE645A001B X-Rspamd-Server: rspam03 X-Stat-Signature: ebx7d58gntea4ppuaqdib53ksbp8dgjq X-HE-Tag: 1760439777-305085 X-HE-Meta: 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 LdEDsZeM kyr7Z4B3jIeRCXoDSSAnheJRocGfkhVfr4fzhY9wcCmI8NPeVx9m2ngd/HTq5oFWqO3DhToXSvLdNojeJYtt4IooGecsCmWksApzflvURva5XqY+I8mbbyREW7yoftWEJxwwRN5l+QcnsIeR9pD9vr9dkOhhfUhHYVkK5bBXPcYGibHp5TSAqsOeqn4RwscZdcUNTAnic/NH+18+hdfjkG/LVWz9JwY/gTyDcTJhBJHnwIXA= 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 13/10/25 17:01, Sean Christopherson wrote: > On Fri, Oct 10, 2025, Valentin Schneider wrote: >> Later commits will cause objtool to warn about static keys being used in >> .noinstr sections in order to safely defer instruction patching IPIs >> targeted at NOHZ_FULL CPUs. >> >> These keys are used in .noinstr code, and can be modified at runtime >> (/proc/kernel/vmx* write). However it is not expected that they will be >> flipped during latency-sensitive operations, and thus shouldn't be a source >> of interference wrt the text patching IPI. >> >> Mark it to let objtool know not to warn about it. > > Can you elaborate in the changelog on what will happen if the key is toggle? > IIUC, smp_text_poke_batch_finish() will force IPIs if noinstr code is being > patched. Right! > Even just a small footnote like this: > > Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is > being patched, i.e. this is purely about silencing objtool warnings. > > to make it clear that there's no bug/race being introduced. Good point. How about: """ Later commits will cause objtool to warn about static keys being used in .noinstr sections in order to safely defer instruction patching IPIs targeted at NOHZ_FULL CPUs. The VMX keys are used in .noinstr code, and can be modified at runtime (/proc/kernel/vmx* write). However it is not expected that they will be flipped during latency-sensitive operations, and thus shouldn't be a source of interference for NOHZ_FULL CPUs wrt the text patching IPI. Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is being patched, i.e. this is purely to tell objtool we're okay with updates to that key causing IPIs and to silence the associated objtool warning. """