From nobody Fri Dec 19 14:12:38 2025 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 43C2C26A0B9 for ; Fri, 10 Oct 2025 15:48:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760111306; cv=none; b=HpDXOo80JFl+/CJpElJxYXKhfSTupLViy1mXWAXYqnieakD9S6PS0UlGASsFdG+FdhMk0Y0OzR3XY+XwvVIqbNJTkm9ls6UtY6kKq/Q6ETLESQrHz9XVedgbL8mdozQeiwo2rA/l8g6cB2kFWZlCAwMNZ7Q59cD6vmqVxbRqPWY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760111306; c=relaxed/simple; bh=/vZYtaWzNZHLvAn1E8dpP5gg037nYFmt++kBJSi+VPM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=K6YaKiE+09xdmp8pEJWpjrBiSzDkkdh6f135oZZtriGA8b58RE1k3U6lzTvP7cLRvYA6dCAAs1gWZZ6AH+BIVZZJ9iY9AZdNOYQBit0/WpWSmdMSwIh8I1KheI/yeyAnzr3MCBj08kfLt0AYgJS5VMg7cj8GSTT90yz8BjUngXk= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=LljsPOCm; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="LljsPOCm" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1760111304; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=BS3ds8Up0Gh0Vcw12QbhifbfMtExSe/6UKJ7yZRlT/s=; b=LljsPOCmEHkKjC8MCu9oHHMxDJIyWkoDrheYbrKHq89H07Av5GQow6S/rmhNaW8NM/S2xN 7moD1G4Juqsy+lYlMSONXEKG95Qlk1/5jW+I/pWRneri+3I4eEbNuiC7JaQGeTOZYKBUIH TIAo89LSoXO2V3MC1RUzESzsDLGCMdo= Received: from mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-227-mm0n-4RTPUCAn9uW9vCAGg-1; Fri, 10 Oct 2025 11:48:23 -0400 X-MC-Unique: mm0n-4RTPUCAn9uW9vCAGg-1 X-Mimecast-MFC-AGG-ID: mm0n-4RTPUCAn9uW9vCAGg_1760111296 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 566A519560B0; Fri, 10 Oct 2025 15:48:16 +0000 (UTC) Received: from vschneid-thinkpadt14sgen2i.remote.csb (unknown [10.45.224.29]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id F0D471800578; Fri, 10 Oct 2025 15:48:01 +0000 (UTC) From: Valentin Schneider To: linux-kernel@vger.kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org, x86@kernel.org, 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" , 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Content-Type: text/plain; charset="utf-8" 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