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 X-Spam-Level: X-Spam-Status: No, score=-8.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BE318C3A5A2 for ; Thu, 22 Aug 2019 10:46:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 94B8D206B7 for ; Thu, 22 Aug 2019 10:46:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732349AbfHVKqd (ORCPT ); Thu, 22 Aug 2019 06:46:33 -0400 Received: from ozlabs.org ([203.11.71.1]:52375 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730333AbfHVKqd (ORCPT ); Thu, 22 Aug 2019 06:46:33 -0400 Received: by ozlabs.org (Postfix, from userid 1034) id 46Dh6H04Gkz9sNC; Thu, 22 Aug 2019 20:46:30 +1000 (AEST) X-powerpc-patch-notification: thanks X-powerpc-patch-commit: 959c5d5134786b4988b6fdd08e444aa67d1667ed In-Reply-To: <20190813100349.GD9567@blackberry> To: Paul Mackerras , linuxppc-dev@ozlabs.org, kvm@vger.kernel.org From: Michael Ellerman Cc: kvm-ppc@vger.kernel.org, David Gibson Subject: Re: [PATCH v2 1/3] KVM: PPC: Book3S HV: Fix race in re-enabling XIVE escalation interrupts Message-Id: <46Dh6H04Gkz9sNC@ozlabs.org> Date: Thu, 22 Aug 2019 20:46:30 +1000 (AEST) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, 2019-08-13 at 10:03:49 UTC, Paul Mackerras wrote: > Escalation interrupts are interrupts sent to the host by the XIVE > hardware when it has an interrupt to deliver to a guest VCPU but that > VCPU is not running anywhere in the system. Hence we disable the > escalation interrupt for the VCPU being run when we enter the guest > and re-enable it when the guest does an H_CEDE hypercall indicating > it is idle. > > It is possible that an escalation interrupt gets generated just as we > are entering the guest. In that case the escalation interrupt may be > using a queue entry in one of the interrupt queues, and that queue > entry may not have been processed when the guest exits with an H_CEDE. > The existing entry code detects this situation and does not clear the > vcpu->arch.xive_esc_on flag as an indication that there is a pending > queue entry (if the queue entry gets processed, xive_esc_irq() will > clear the flag). There is a comment in the code saying that if the > flag is still set on H_CEDE, we have to abort the cede rather than > re-enabling the escalation interrupt, lest we end up with two > occurrences of the escalation interrupt in the interrupt queue. > > However, the exit code doesn't do that; it aborts the cede in the sense > that vcpu->arch.ceded gets cleared, but it still enables the escalation > interrupt by setting the source's PQ bits to 00. Instead we need to > set the PQ bits to 10, indicating that an interrupt has been triggered. > We also need to avoid setting vcpu->arch.xive_esc_on in this case > (i.e. vcpu->arch.xive_esc_on seen to be set on H_CEDE) because > xive_esc_irq() will run at some point and clear it, and if we race with > that we may end up with an incorrect result (i.e. xive_esc_on set when > the escalation interrupt has just been handled). > > It is extremely unlikely that having two queue entries would cause > observable problems; theoretically it could cause queue overflow, but > the CPU would have to have thousands of interrupts targetted to it for > that to be possible. However, this fix will also make it possible to > determine accurately whether there is an unhandled escalation > interrupt in the queue, which will be needed by the following patch. > > Cc: stable@vger.kernel.org # v4.16+ > Fixes: 9b9b13a6d153 ("KVM: PPC: Book3S HV: Keep XIVE escalation interrupt masked unless ceded") > Signed-off-by: Paul Mackerras Applied to powerpc topic/ppc-kvm, thanks. https://git.kernel.org/powerpc/c/959c5d5134786b4988b6fdd08e444aa67d1667ed cheers