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[213.44.141.166]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-438bd48a145sm128093025e9.16.2025.01.27.03.17.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Jan 2025 03:17:05 -0800 (PST) From: Valentin Schneider To: Frederic Weisbecker Cc: linux-kernel@vger.kernel.org, x86@kernel.org, virtualization@lists.linux.dev, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org, linux-perf-users@vger.kernel.org, xen-devel@lists.xenproject.org, kvm@vger.kernel.org, linux-arch@vger.kernel.org, rcu@vger.kernel.org, linux-hardening@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, bpf@vger.kernel.org, bcm-kernel-feedback-list@broadcom.com, Juergen Gross , Ajay Kaher , Alexey Makhalov , Russell King , Catalin Marinas , Will Deacon , Huacai Chen , WANG Xuerui , Paul Walmsley , Palmer Dabbelt , Albert Ou , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Peter Zijlstra , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , "Liang, Kan" , Boris Ostrovsky , Josh Poimboeuf , Pawan Gupta , Sean Christopherson , Paolo Bonzini , Andy Lutomirski , Arnd Bergmann , "Paul E. McKenney" , Jason Baron , Steven Rostedt , Ard Biesheuvel , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Juri Lelli , Clark Williams , Yair Podemsky , Tomas Glozar , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman , Kees Cook , Andrew Morton , Christoph Hellwig , Shuah Khan , Sami Tolvanen , Miguel Ojeda , Alice Ryhl , "Mike Rapoport (Microsoft)" , Samuel Holland , Rong Xu , Nicolas Saenz Julienne , Geert Uytterhoeven , Yosry Ahmed , "Kirill A. Shutemov" , "Masami Hiramatsu (Google)" , Jinghao Jia , Luis Chamberlain , Randy Dunlap , Tiezhu Yang Subject: Re: [PATCH v4 22/30] context_tracking: Exit CT_STATE_IDLE upon irq/nmi entry In-Reply-To: References: <20250114175143.81438-1-vschneid@redhat.com> <20250114175143.81438-23-vschneid@redhat.com> Date: Mon, 27 Jan 2025 12:17:03 +0100 Message-ID: MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: P1ZwaTgV98wJWKoNwTVmApijhLWBDR5Fh4-JFY9FQG0_1737976627 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2BC0080014 X-Stat-Signature: 74e8hmpunssi17z5ci3aj5kr8zhedxoa X-Rspam-User: X-HE-Tag: 1737976630-380670 X-HE-Meta: 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 GkCRqBzF vN7x/ttEz5ypjwF/f1NquhVBziZew/38AcYrzar2lsOd2p2c8QcyvBcZzIWPDeTTRFNF9bQVlTPjU2fgHr/rStmNm4gH436MpOnMdnHqvEHFhuZGWCtDeCAEij02FcTUbwY6pZf7yh8QG+iJkG39CRTcVKlvoK8Qx7xxKR2lV1QrDRp1R9y6tGOl+gYFfGBWKh2JuAU8HNSUKdW9RDUb2uY/PIxzFRw4Uf1M72PC7Ub3aN+Jy8cFk0hvSHW5Xs+9w/Mr1oYUmYRL2gjtpS1p2A4TRDhBoCr/ETD2pKmVRr736MsPzZ5qJuKlWTs0BQgQcqjSiGp/kaBj0JhurkSWFaU8bVLYmDWm80pjoEt+RGmPF8/ll2t/Y5Yio2NSuiIXjTHSGlVpp3Cok6FeLWU/hSA/+Ga3oKKF1jxdwy10wEXnmaJWNNAZeUKPIzOb86UQJcsLYHTHVoili3Tjm+F5QGj8FxzTjrYEMY8jF 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 22/01/25 01:22, Frederic Weisbecker wrote: > Le Tue, Jan 14, 2025 at 06:51:35PM +0100, Valentin Schneider a =C3=A9crit= : >> ct_nmi_{enter, exit}() only touches the RCU watching counter and doesn't >> modify the actual CT state part context_tracking.state. This means that >> upon receiving an IRQ when idle, the CT_STATE_IDLE->CT_STATE_KERNEL >> transition only happens in ct_idle_exit(). >> >> One can note that ct_nmi_enter() can only ever be entered with the CT st= ate >> as either CT_STATE_KERNEL or CT_STATE_IDLE, as an IRQ/NMI happenning in = the >> CT_STATE_USER or CT_STATE_GUEST states will be routed down to ct_user_ex= it(). > > Are you sure? An NMI can fire between guest_state_enter_irqoff() and > __svm_vcpu_run(). Urgh, you're quite right. > And NMIs interrupting userspace don't call > enter_from_user_mode(). In fact they don't call irqentry_enter_from_user_= mode() > like regular IRQs but irqentry_nmi_enter() instead. Well that's for archs > implementing common entry code, I can't speak for the others. > That I didn't realize, so thank you for pointing it out. Having another look now, I mistook DEFINE_IDTENTRY_RAW(exc_int3) for the general case when it really isn't :( > Unifying the behaviour between user and idle such that the IRQs/NMIs exit= the > CT_STATE can be interesting but I fear this may not come for free. You wo= uld > need to save the old state on IRQ/NMI entry and restore it on exit. > That's what I tried to avoid, but it sounds like there's no nice way around= it. > Do we really need it? > Well, my problem with not doing IDLE->KERNEL transitions on IRQ/NMI is that this leads the IPI deferral logic to observe a technically-out-of-sync sate for remote CPUs. Consider: CPUx CPUy state :=3D CT_STATE_IDLE ... ~>IRQ ... ct_nmi_enter() [in the kernel proper by now] text_poke_bp_batch() ct_set_cpu_work(CPUy, CT_WORK_SYNC) READ CPUy ct->state `-> CT_IDLE_STATE `-> defer IPI I thought this meant I would need to throw out the "defer IPIs if CPU is idle" part, but AIUI this also affects CT_STATE_USER and CT_STATE_GUEST, which is a bummer :(