From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 58C97C433EF for ; Mon, 10 Jan 2022 10:51:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244287AbiAJKvW (ORCPT ); Mon, 10 Jan 2022 05:51:22 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:53406 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244251AbiAJKuu (ORCPT ); Mon, 10 Jan 2022 05:50:50 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1641811850; 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=SbyEAGXm6uVAR10E6kW8QyvfQbDxdraL/FSOR9ycncM=; b=DWfh2sHgOTWe5PHKjpQ367wogut3R3UTewg47/sRHT9R2h1O/CeEzIXKHVDhmrRtU4TVcz EPkvQxQdmWkBBv1QWvN1zoyk2k+5vPir9VQmz1SpiI8ZY9h+YuNEhAu2maR/bEgmXpEKKD kBcHM/0tFq2C9NiZBW7mboyZVFyi8oM= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-632-mXwteg9RNxqsWK8GN-LJ1Q-1; Mon, 10 Jan 2022 05:50:49 -0500 X-MC-Unique: mXwteg9RNxqsWK8GN-LJ1Q-1 Received: by mail-wr1-f69.google.com with SMTP id k4-20020adfc704000000b001a32d86a772so4059598wrg.5 for ; Mon, 10 Jan 2022 02:50:48 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=SbyEAGXm6uVAR10E6kW8QyvfQbDxdraL/FSOR9ycncM=; b=zXTzhEUADRbQ6yC+4GGQ1pVCvDcl333rigooiqIbNs+emK2TglBB6p2daIpAfFbe3U 4L67xsjJookuaDENgphMD9nkPhGSgTrXBtMuLehJixTSKBBNQcKQUCx51YKGrHBcAPNf Me0rhXWkzr0mmIpXVsfDjj7fXIdrKuGp32n2MEgcrbZdf+k20KpGFi7cBqRwZqJYODBq 9dVTH91T93w0UvJurPVsqRE9OVbYkT+ts3BG6OGwx2ZeCjVoI6w7ojKkD/VxHjCK04rD UeN+lytrnoMjf0EGVQbhih0/nJrOu6sHrDzz6VgA6oBHw9ZpRyGyccUz66kzGha4JTic 48SA== X-Gm-Message-State: AOAM530VZBTPKwNDt4IRplwzASgrB44nCaQQpDotgyxb66qd0DcnxaQN CNVvZyL3FvuaUzrDkJ1gemzVabAqDxt65iIfsiRA6u5GfEM2GM0x3hKgOej+nTHlb2QA0jMnnY8 iRN/ZOKJoaSCE6AYimfagFpfx X-Received: by 2002:a7b:c305:: with SMTP id k5mr21416715wmj.144.1641811847843; Mon, 10 Jan 2022 02:50:47 -0800 (PST) X-Google-Smtp-Source: ABdhPJysN9EkPLfD6O/v+iNsqBZ2ymYyZycxdVBE8rH21g120evtaf2rhvzsHLMdH6me9L3zGbBSpA== X-Received: by 2002:a7b:c305:: with SMTP id k5mr21416701wmj.144.1641811847602; Mon, 10 Jan 2022 02:50:47 -0800 (PST) Received: from vian.redhat.com ([2a0c:5a80:1e09:d600:637e:7f1b:a4a0:fc92]) by smtp.gmail.com with ESMTPSA id r1sm7051224wrz.30.2022.01.10.02.50.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Jan 2022 02:50:47 -0800 (PST) From: Nicolas Saenz Julienne To: tglx@linutronix.de, mark.rutland@arm.com, paulmck@kernel.org Cc: rostedt@goodmis.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org, peterz@infradead.org, mtosatti@redhat.com, frederic@kernel.org, corbet@lwn.net, Nicolas Saenz Julienne Subject: [PATCH v4 2/2] Documentation: core-api: entry: Add comments about nesting Date: Mon, 10 Jan 2022 11:50:44 +0100 Message-Id: <20220110105044.94423-2-nsaenzju@redhat.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20220110105044.94423-1-nsaenzju@redhat.com> References: <20220110105044.94423-1-nsaenzju@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The topic of nesting and reentrancy in the context of early entry code hasn't been addressed so far. So do it. Signed-off-by: Nicolas Saenz Julienne --- Changes since v3: - Introduce Paul's rewording suggestions Documentation/core-api/entry.rst | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst index c6f8e22c88fe..e12f22ab33c7 100644 --- a/Documentation/core-api/entry.rst +++ b/Documentation/core-api/entry.rst @@ -105,6 +105,8 @@ has to do extra work between the various steps. In such cases it has to ensure that enter_from_user_mode() is called first on entry and exit_to_user_mode() is called last on exit. +Do not nest syscalls. Nested systcalls will cause RCU and/or context tracking +to print a warning. KVM --- @@ -121,6 +123,8 @@ Task work handling is done separately for guest at the boundary of the vcpu_run() loop via xfer_to_guest_mode_handle_work() which is a subset of the work handled on return to user space. +Do not nest KVM entry/exit transitions because doing so is nonsensical. + Interrupts and regular exceptions --------------------------------- @@ -180,6 +184,16 @@ before it handles soft interrupts, whose handlers must run in BH context rather than irq-disabled context. In addition, irqentry_exit() might schedule, which also requires that HARDIRQ_OFFSET has been removed from the preemption count. +Even though interrupt handlers are expected to run with local interrupts +disabled, interrupt nesting is common from an entry/exit perspective. For +example, softirq handling happens within an irqentry_{enter,exit}() block with +local interrupts enabled. Also, although uncommon, nothing prevents an +interrupt handler from re-enabling interrupts. + +Interrupt entry/exit code doesn't strictly need to handle reentrancy, since it +runs with local interrupts disabled. But NMIs can happen anytime, and a lot of +the entry code is shared between the two. + NMI and NMI-like exceptions --------------------------- @@ -259,3 +273,7 @@ and for e.g. a debug exception it can look like this: There is no combined irqentry_nmi_if_kernel() function available as the above cannot be handled in an exception-agnostic way. + +NMIs can happen in any context. For example, an NMI-like exception triggered +while handling an NMI. So NMI entry code has to be reentrant and state updates +need to handle nesting. -- 2.34.1