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Wed, 19 Nov 2025 09:31:51 -0500 (EST) X-Mailer: MessagingEngine.com Webmail Interface MIME-Version: 1.0 X-ThreadId: A2ZwPDH9FoLc Date: Wed, 19 Nov 2025 06:31:30 -0800 From: "Andy Lutomirski" To: "Valentin Schneider" , "Linux Kernel Mailing List" , linux-mm@kvack.org, rcu@vger.kernel.org, "the arch/x86 maintainers" , 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 Cc: "Thomas Gleixner" , "Ingo Molnar" , "Borislav Petkov" , "Dave Hansen" , "H. Peter Anvin" , "Peter Zijlstra (Intel)" , "Arnaldo Carvalho de Melo" , "Josh Poimboeuf" , "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" , "Shrikanth Hegde" Message-Id: <65ae9404-5d7d-42a3-969e-7e2ceb56c433@app.fastmail.com> In-Reply-To: <20251114151428.1064524-9-vschneid@redhat.com> References: <20251114150133.1056710-1-vschneid@redhat.com> <20251114151428.1064524-9-vschneid@redhat.com> Subject: Re: [RFC PATCH v7 29/31] x86/mm/pti: Implement a TLB flush immediately after a switch to kernel CR3 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: C5DF34001C X-Stat-Signature: u6q45js7u9bfai73s8gx7t5onkcbwcz6 X-Rspam-User: X-HE-Tag: 1763562715-211200 X-HE-Meta: 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 Vow== 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 Fri, Nov 14, 2025, at 7:14 AM, Valentin Schneider wrote: > Deferring kernel range TLB flushes requires the guarantee that upon > entering the kernel, no stale entry may be accessed. The simplest way = to > provide such a guarantee is to issue an unconditional flush upon switc= hing > to the kernel CR3, as this is the pivoting point where such stale entr= ies > may be accessed. > Doing this together with the PTI CR3 switch has no actual benefit: MOV C= R3 doesn=E2=80=99t flush global pages. And doing this in asm is pretty g= ross. We don=E2=80=99t even get a free sync_core() out of it because IN= VPCID is not documented as being serializing. Why can=E2=80=99t we do it in C? What=E2=80=99s the actual risk? In or= der to trip over a stale TLB entry, we would need to deference a pointer= to newly allocated kernel virtual memory that was not valid prior to ou= r entry into user mode. I can imagine BPF doing this, but plain noinstr = C in the entry path? Especially noinstr C *that has RCU disabled*? We = already can=E2=80=99t follow an RCU pointer, and ISTM the only style of = kernel code that might do this would use RCU to protect the pointer, and= we are already doomed if we follow an RCU pointer to any sort of memory. We do need to watch out for NMI/MCE hitting before we flush.