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Mon, 10 Feb 2025 14:09:33 -0800 (PST) MIME-Version: 1.0 References: <20250114175143.81438-1-vschneid@redhat.com> <20250114175143.81438-30-vschneid@redhat.com> In-Reply-To: From: Jann Horn Date: Mon, 10 Feb 2025 23:08:55 +0100 X-Gm-Features: AWEUYZmtdNVDV_xq3vvo5TVQ7aaS79EFzdycTCfl72XGpztchND9QHjN9zcxWNo Message-ID: Subject: Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs To: Valentin Schneider 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 , Frederic Weisbecker , "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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 4C42BC0007 X-Stat-Signature: 58gx3h6iyf51thz4mnajztppjkn77ub5 X-HE-Tag: 1739225375-165207 X-HE-Meta: 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 vxIZmJic cv346jfcjwsVzbjANQiVi3bh7JdiI2mIIj+HLeLD/uIRY5R6wZ5UdFoR0wfTFjJ9YLFQPfewfF/AsMOA6V6vC5wJohpxNhRRreRgJaimvQ1pWazhnVA/BILCSsUa+Qotc3/SFqXq0UA5YvSfU0EMvMGjfEjw6bKqLGO7kLEfb4vdWi+LmLWP+aAEKAaQ/9RjVZ+Rr7zDcQMydqEz2lKbPRXu9u6vtOjCTj7OYzAi8wX8j+q+n4LAhnDY9Ouw1pLt7SNqBrpelaVzFNxdvDGawIR4iaTpfNdywBBd872oo12K9edzLP0ceybuoqALsUiLyQ6sL2MJKPzDYuJUbJdK8kKPhUe/84np/WCEYYv9Aw2GJieXcclGf9FWFXCniFsE4egmRzMNSCivcTQY= 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 Mon, Feb 10, 2025 at 7:36=E2=80=AFPM Valentin Schneider wrote: > What if isolated CPUs unconditionally did a TLBi as late as possible in > the stack right before returning to userspace? This would mean that upon > re-entering the kernel, an isolated CPU's TLB wouldn't contain any kernel > range translation - with the exception of whatever lies between the > last-minute flush and the actual userspace entry, which should be feasibl= e > to vet? Then AFAICT there wouldn't be any work/flush to defer, the IPI > could be entirely silenced if it targets an isolated CPU. Two issues with that: 1. I think the "Common not Private" feature Will Deacon referred to is incompatible with this idea: says "When the CnP bit is set, the software promises to use the ASIDs and VMIDs in the same way on all processors, which allows the TLB entries that are created by one processor to be used by another" 2. It's wrong to assume that TLB entries are only populated for addresses you access - thanks to speculative execution, you have to assume that the CPU might be populating random TLB entries all over the place.