Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:6 as permitted sender) client-ip=2620:137:e000::3:6;
Date:   Mon, 2 Oct 2023 13:57:18 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Dapeng Mi <dapeng1.mi@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Kan Liang <kan.liang@linux.intel.com>,
        Like Xu <likexu@tencent.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>, kvm@vger.kernel.org,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhang Xiong <xiong.y.zhang@intel.com>,
        Lv Zhiyuan <zhiyuan.lv@intel.com>,
        Yang Weijiang <weijiang.yang@intel.com>,
        Dapeng Mi <dapeng1.mi@intel.com>,
        Jim Mattson <jmattson@google.com>,
        David Dunn <daviddunn@google.com>,
        Mingwei Zhang <mizhang@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics
 event
Message-ID: <20231002115718.GB13957@noisy.programming.kicks-ass.net>
References: <20230927033124.1226509-1-dapeng1.mi@linux.intel.com>
 <20230927033124.1226509-8-dapeng1.mi@linux.intel.com>
 <20230927113312.GD21810@noisy.programming.kicks-ass.net>
 <ZRRl6y1GL-7RM63x@google.com>
 <20230929115344.GE6282@noisy.programming.kicks-ass.net>
 <ZRbxb15Opa2_AusF@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZRbxb15Opa2_AusF@google.com>
Precedence: bulk

On Fri, Sep 29, 2023 at 03:46:55PM +0000, Sean Christopherson wrote:

> > I will firmly reject anything that takes the PMU away from the host
> > entirely through.
> 
> Why?  What is so wrong with supporting use cases where the platform owner *wants*
> to give up host PMU and NMI watchdog functionality?  If disabling host PMU usage
> were complex, highly invasive, and/or difficult to maintain, then I can understand
> the pushback.  

Because it sucks.

You're forcing people to choose between no host PMU or a slow guest PMU.
And that's simply not a sane choice for most people -- worse it's not a
choice based in technical reality.

It's a choice out of lazyness, disabling host PMU is not a requirement
for pass-through. 

Like I wrote, all we need to do is ensure vCPU tasks will never have a
perf-event scheduled that covers guest mode. Currently this would be
achievable by having event creation for both:

 - CPU events without attr::exclude_guest=1, and
 - task events for vCPU task of interest without attr::exclude_guest=1

error with -EBUSY or something.

This ensures there are no events active for those vCPU tasks at VMENTER
time and you can haz pass-through.