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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 , Shrikanth Hegde Subject: [RFC PATCH v7 31/31] x86/entry: Add an option to coalesce TLB flushes Date: Fri, 14 Nov 2025 16:14:28 +0100 Message-ID: <20251114151428.1064524-11-vschneid@redhat.com> In-Reply-To: <20251114150133.1056710-1-vschneid@redhat.com> References: <20251114150133.1056710-1-vschneid@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 7CE27140017 X-Stat-Signature: cpkc8dkxskdeg71ubfdnb4hyygunxtkt X-Rspam-User: X-HE-Tag: 1763133464-3003 X-HE-Meta: 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 ab2bqdQk rjwt8mr2Yl9DfeWFxbH6YAB7uc7x82FLQt6Tzp9NK7oqfXNpbhzk+wTVqOv97rBwLUiGqZLszPpJH5GEjXh6vDJNAMXTYTvfkCTHj2cNBnnfLuVNPk9wrfykVi6euGJOV3mQWeXaIyFBsTvusb839lAbV/1I0hFQy5EX7bzkMP4nTkqwLVu2u4F/L8Bs8Q2fJAxrJA6F/ckPIqFX9pG4C3oQGVot1n3p1SQeUwySmYJ3btffVNHDhCjqEtAUGF9a0KB65rfGDCEtbVnKWgVO3QM6bWbH9ArWDFZ3ZKdwRteo6x3r9/mjEjSbYpcoxCMUhcmaQ5jTMqhC5iYXnU5Nf6N5Arou4b5UIwmAE1QvcUDPOMhSM5dF5zdIonSO3r92xyI9vYM4qOlai5QhcSteWarnJ/TQ+x+rckgngTHwnOIoBPQk= 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 fa9229c0e0939..04f9d6496bbbc 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -2189,6 +2189,23 @@ 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