From mboxrd@z Thu Jan 1 00:00:00 1970 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 E3FC432E157 for ; Fri, 14 Nov 2025 15:17:39 +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=1763133461; cv=none; b=cXgrHlfFszvAtRQNSt6NdbSFaj+AzEGyYbzCI3V0+cVNKVqCCC3cAhCGfJty0kef6y+gx9+ymqhhhailXfj3WJIvzEsqRx0Bf1trt+h2ySLWhDz6Glt+UmCdjBNnsme25l/1spiwXYB+HuBcyT5Zu8KT6wVXLYHLWNTV956W1LI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763133461; c=relaxed/simple; bh=k4QzhlWoFW0nKlX+gCLSWARw5LDdpUWLbvAWfPBN+UA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Dyy8iZexTV541Ie2y92Sc2H4LALRy21j2fU12epJ2pf/W7a549Of/LA8NiK9oY9jTIfEn0ForTVye4/Ck7P/YcP3txVfwA+Km3FSf8UeG8m0R42qJ+HImpJLxqtiafO9JKUS3l26cfQPmRNwMkmTT/YYeqTaHLPuV5Z9D+CsPYk= 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=H4ZHlWAt; 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="H4ZHlWAt" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1763133459; 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=YHbOf6w5rlj656BCFSOxLzgqhHO7ycdTLJssDuPvEBo=; b=H4ZHlWAtFWfqQpYAaQE+D1y4t71cCMT7IpvWhzWTk6HqrlaASftXtFZOfUTbuonwpP60Wi cpxxhRz0Jr+2YYXLWPtzsmCT+7q6MgdDGoLEK73gHZ1CL0O6iNq1JOCY+W9nToXSyIMqWi RIunFr32P4ZRXEZEbpksmBRYUOFHwIg= Received: from mx-prod-mc-05.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-531--Ii0M5UDM1-LvTUINdpoDg-1; Fri, 14 Nov 2025 10:17:37 -0500 X-MC-Unique: -Ii0M5UDM1-LvTUINdpoDg-1 X-Mimecast-MFC-AGG-ID: -Ii0M5UDM1-LvTUINdpoDg_1763133453 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2DC7C1956096; Fri, 14 Nov 2025 15:17:33 +0000 (UTC) Received: from vschneid-thinkpadt14sgen2i.remote.csb (unknown [10.45.226.10]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A94EB180049F; Fri, 14 Nov 2025 15:17:18 +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 , 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> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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