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 94681C001E0 for ; Wed, 9 Aug 2023 09:22:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232020AbjHIJWo (ORCPT ); Wed, 9 Aug 2023 05:22:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33850 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231996AbjHIJWl (ORCPT ); Wed, 9 Aug 2023 05:22:41 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 4EB9C1FDF; Wed, 9 Aug 2023 02:22:40 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 49989D75; Wed, 9 Aug 2023 02:23:22 -0700 (PDT) Received: from FVFF77S0Q05N (unknown [10.57.3.120]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 455C53F59C; Wed, 9 Aug 2023 02:22:36 -0700 (PDT) Date: Wed, 9 Aug 2023 10:22:25 +0100 From: Mark Rutland To: Oliver Upton Cc: Huang Shijie , maz@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, pbonzini@redhat.com, peterz@infradead.org, ingo@redhat.com, acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@kernel.org, namhyung@kernel.org, irogers@google.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-perf-users@vger.kernel.org, patches@amperecomputing.com, zwang@amperecomputing.com Subject: Re: [PATCH] perf/core: fix the bug in the event multiplexing Message-ID: References: <20230809013953.7692-1-shijie@os.amperecomputing.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 09, 2023 at 08:25:07AM +0000, Oliver Upton wrote: > Hi Huang, > > On Wed, Aug 09, 2023 at 09:39:53AM +0800, Huang Shijie wrote: > > 2.) Root cause. > > There is only 7 counters in my arm64 platform: > > (one cycle counter) + (6 normal counters) > > > > In 1.3 above, we will use 10 event counters. > > Since we only have 7 counters, the perf core will trigger > > event multiplexing in hrtimer: > > merge_sched_in() -->perf_mux_hrtimer_restart() --> > > perf_rotate_context(). > > > > In the perf_rotate_context(), it does not restore some PMU registers > > as context_switch() does. In context_switch(): > > kvm_sched_in() --> kvm_vcpu_pmu_restore_guest() > > kvm_sched_out() --> kvm_vcpu_pmu_restore_host() > > > > So we got wrong result. > > This is a rather vague description of the problem. AFAICT, the > issue here is on VHE systems we wind up getting the EL0 count > enable/disable bits backwards when entering the guest, which is > corroborated by the data you have below. Yep; IIUC the issue here is that when we take an IRQ from a guest and reprogram the PMU in the IRQ handler, the IRQ handler will program the PMU with appropriate host/guest/user/etc filters for a *host* context, and then we'll return back into the guest without reconfigurign the event filtering for a *guest* context. That can happen for perf_rotate_context(), or when we install an event into a running context, as that'll happen via an IPI. > > +void arch_perf_rotate_pmu_set(void) > > +{ > > + if (is_guest()) > > + kvm_vcpu_pmu_restore_guest(NULL); > > + else > > + kvm_vcpu_pmu_restore_host(NULL); > > +} > > + > > This sort of hook is rather nasty, and I'd strongly prefer a solution > that's confined to KVM. I don't think the !is_guest() branch is > necessary at all. Regardless of how the pmu context is changed, we need > to go through vcpu_put() before getting back out to userspace. > > We can check for a running vCPU (ick) from kvm_set_pmu_events() and either > do the EL0 bit flip there or make a request on the vCPU to call > kvm_vcpu_pmu_restore_guest() immediately before reentering the guest. > I'm slightly leaning towards the latter, unless anyone has a better idea > here. The latter sounds reasonable to me. I suspect we need to take special care here to make sure we leave *all* events in a good state when re-entering the guest or if we get to kvm_sched_out() after *removing* an event via an IPI -- it'd be easy to mess either case up and leave some events in a bad state. Thanks, Mark.