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=-9.0 required=3.0 tests=BAYES_00,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 F1C72C433DB for ; Mon, 4 Jan 2021 18:27:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C5E1621D94 for ; Mon, 4 Jan 2021 18:27:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726599AbhADS1Z (ORCPT ); Mon, 4 Jan 2021 13:27:25 -0500 Received: from mail.kernel.org ([198.145.29.99]:44224 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726396AbhADS1Z (ORCPT ); Mon, 4 Jan 2021 13:27:25 -0500 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 1EDFD2075E; Mon, 4 Jan 2021 18:26:44 +0000 (UTC) Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.94) (envelope-from ) id 1kwUZ0-005HbV-06; Mon, 04 Jan 2021 18:26:42 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Mon, 04 Jan 2021 18:26:41 +0000 From: Marc Zyngier To: Qian Cai Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kernel-team@android.com, Stephen Rothwell , Linux Next Mailing List , Alexandru Elisei Subject: Re: [PATCH] KVM: arm64: Don't access PMCR_EL0 when no PMU is available In-Reply-To: References: <20201210083059.1277162-1-maz@kernel.org> <703e1b5f2db1631e8f9c55619909fe66eb069f25.camel@redhat.com> <579c839a0016107af66e704f147f9814@kernel.org> User-Agent: Roundcube Webmail/1.4.9 Message-ID: X-Sender: maz@kernel.org X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: qcai@redhat.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kernel-team@android.com, sfr@canb.auug.org.au, linux-next@vger.kernel.org, alexandru.elisei@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: linux-next@vger.kernel.org On 2021-01-04 18:20, Qian Cai wrote: > On Mon, 2021-01-04 at 16:27 +0000, Marc Zyngier wrote: >> On 2021-01-04 16:22, Qian Cai wrote: >> > On Mon, 2021-01-04 at 16:08 +0000, Marc Zyngier wrote: >> > > On 2021-01-04 15:47, Qian Cai wrote: >> > > > On Thu, 2020-12-10 at 08:30 +0000, Marc Zyngier wrote: >> > > > > We reset the guest's view of PMCR_EL0 unconditionally, based on >> > > > > the host's view of this register. It is however legal for an >> > > > > imnplementation not to provide any PMU, resulting in an UNDEF. >> > > > > >> > > > > The obvious fix is to skip the reset of this shadow register >> > > > > when no PMU is available, sidestepping the issue entirely. >> > > > > If no PMU is available, the guest is not able to request >> > > > > a virtual PMU anyway, so not doing nothing is the right thing >> > > > > to do! >> > > > > >> > > > > It is unlikely that this bug can hit any HW implementation >> > > > > though, as they all provide a PMU. It has been found using nested >> > > > > virt with the host KVM not implementing the PMU itself. >> > > > > >> > > > > Fixes: ab9468340d2bc ("arm64: KVM: Add access handler for PMCR >> > > > > register") >> > > > > Signed-off-by: Marc Zyngier >> > > > >> > > > Reverting this commit on the top of today's linux-next fixed a qemu-kvm >> > > > coredump >> > > > issue on TX2 while starting a guest. >> > > > >> > > > - host kernel .config: >> > > > https://cailca.coding.net/public/linux/mm/git/files/master/arm64.config >> > > > >> > > > # /usr/libexec/qemu-kvm -name ubuntu-20.04-server-cloudimg -cpu host >> > > > -smp 2 -m 2g >> > > > -drive >> > > > if=none,format=qcow2,file=./ubuntu-20.04-server-cloudimg.qcow2,id=hd >> > > > -device virtio-scsi -device scsi-hd,drive=hd -cdrom >> > > > ./ubuntu-20.04-server-cloudimg.iso >> > > > -bios /usr/share/AAVMF/AAVMF_CODE.fd -M gic-version=host -nographic >> > > > -nic user,model=virtio,hostfwd=tcp::2222-:22 >> > > > >> > > > qemu-kvm: /builddir/build/BUILD/qemu-4.2.0/target/arm/helper.c:1812: >> > > > pmevcntr_rawwrite: Assertion `counter < pmu_num_counters(env)' failed. >> > > >> > > You don't have KVM_ARM_PMU selected in your config, so QEMU cannot >> > > access the PMU registers, and no counters are exposed. >> > >> > Well, isn't it the rule that don't break the userspace? qemu works fine >> > with >> > KVM_ARM_PMU=n until this commit. >> >> No, it doesn't "work fine". It gets random data that potentially makes >> no sense, >> depending on the HW this runs on. >> >> Now, userspace tells you that your kernel is misconfigured. I see it >> as >> an improvement. > > Marc, do you suggest that CONFIG_KVM=y should select KVM_ARM_PMU=y > then? > Otherwise, this is rather difficult for users to figure out and a core > dump with > an implicit error message from qemu is not that helpful. What I'm suggesting is this [1], which is to get rid of KVM_ARM_PMU completely. At least, the kernel configuration will be consistent. Overall, I think there is an issue with KVM exposing more than it should to userspace when no PMU is defined, but I don't think that's the problem you are seeing. M. [1] https://lore.kernel.org/r/20210104172723.2014324-1-maz@kernel.org -- Jazz is not dead. It just smells funny...