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Peter Anvin" , Andy Lutomirski , Peter Zijlstra , 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 Subject: [RFC PATCH v6 29/29] x86/entry: Add an option to coalesce TLB flushes Date: Fri, 10 Oct 2025 17:38:39 +0200 Message-ID: <20251010153839.151763-30-vschneid@redhat.com> In-Reply-To: <20251010153839.151763-1-vschneid@redhat.com> References: <20251010153839.151763-1-vschneid@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Rspamd-Server: rspam05 X-Stat-Signature: 7cdttcmybzzui7ac615usddt4ioitins X-Rspam-User: X-Rspamd-Queue-Id: 2AAFB180003 X-HE-Tag: 1760111306-544378 X-HE-Meta: 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 +N6CHxxr HYmGpk9PHy+IUm0VVSWsrtmgz8MuxwyQPLx+tVAJw2I7QDrbgO3eBFcvcZ9pHiNx1bpyz6F7jcKTmwH4Nq17dH3KxrWM2N9t1exVCIIgxXztYelBhd3/DwBix832OpnvP1f3Aqk9f4xd59gljdr+9uAI3SQtt2ftJtqAU8Y4aLXGVPayXZRWL2s3DLPZK9wx21ReO9RogL6JetZVWNpVfHzXc74wWEQGFeUASEoyoQiZvkJJ+eSs4ee9yBS/aUPHZ1droFr+26lbzt7FrWmhnw85cdlQIl/G2BfFoG0QNZcCt7ul3nI1FU9cbnflPXaX/nGgefETACRv15lUDxc7L+3r81GOwQuhT/Sa4w6W/LUL1HX3b9xaQvU1nAs/6UBJRiCcMhfSlmIkpb1pcUeWYH7/ziOTlGLEpdxBTljXNzVQLYcE= 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: Previous patches have introduced a mechanism to prevent kernel text updates from inducing interference on isolated CPUs. A similar action is required for kernel-range TLB flushes in order to silence the biggest remaining cause of isolated CPU IPI interference. These flushes are mostly caused by vmalloc manipulations - e.g. on x86 with CONFIG_VMAP_STACK, spawning enough processes will easily trigger flushes. Unfortunately, the newly added context_tracking IPI deferral mechanism cannot be leveraged for TLB flushes, as the deferred work would be executed too late. Consider the following execution flow: !interrupt! SWITCH_TO_KERNEL_CR3 // vmalloc range becomes accessible idtentry_func_foo() irqentry_enter() irqentry_enter_from_user_mode() enter_from_user_mode() [...] ct_kernel_enter_state() ct_work_flush() // deferred flush would be done here Since there is no sane way to assert no stale entry is accessed during kernel entry, any code executed between SWITCH_TO_KERNEL_CR3 and ct_work_flush() is at risk of accessing a stale entry. Dave had suggested hacking up something within SWITCH_TO_KERNEL_CR3 itself, which is what has been implemented in the previous patches. Make kernel-range TLB flush deferral available via CONFIG_COALESCE_TLBI. Signed-off-by: Valentin Schneider --- arch/x86/Kconfig | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 3f1557b7acd8f..390e1dbe5d4de 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -2188,6 +2188,23 @@ config ADDRESS_MASKING The capability can be used for efficient address sanitizers (ASAN) implementation and for optimizations in JITs. +config COALESCE_TLBI + def_bool n + prompt "Coalesce kernel TLB flushes for NOHZ-full CPUs" + depends on X86_64 && MITIGATION_PAGE_TABLE_ISOLATION && NO_HZ_FULL + help + TLB flushes for kernel addresses can lead to IPIs being sent to + NOHZ-full CPUs, thus kicking them out of userspace. + + This option coalesces kernel-range TLB flushes for NOHZ-full CPUs into + a single flush executed at kernel entry, right after switching to the + kernel page table. Note that this flush is unconditionnal, even if no + remote flush was issued during the previous userspace execution window. + + This obviously makes the user->kernel transition overhead even worse. + + If unsure, say N. + config HOTPLUG_CPU def_bool y depends on SMP -- 2.51.0