linux-kernel - Re: [PATCH v4 20/38] KVM: x86/pmu: Check if mediated vPMU can intercept rdpmc

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Message-ID: <aJFWK-EiGgH5aiw1@google.com>
Date: Mon, 4 Aug 2025 17:54:03 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Sandipan Das <sandipan.das@....com>, Mingwei Zhang <mizhang@...gle.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com, 
	Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Yongwei Ma <yongwei.ma@...el.com>, Xiong Zhang <xiong.y.zhang@...ux.intel.com>, 
	Jim Mattson <jmattson@...gle.com>, Zide Chen <zide.chen@...el.com>, 
	Eranian Stephane <eranian@...gle.com>, Shukla Manali <Manali.Shukla@....com>, 
	Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 20/38] KVM: x86/pmu: Check if mediated vPMU can
 intercept rdpmc

On Fri, Aug 01, 2025, Sean Christopherson wrote:
> On Wed, Jul 30, 2025, Dapeng Mi wrote:
> > 
> > On 7/30/2025 8:38 AM, Sean Christopherson wrote:
> > > On Tue, Jul 29, 2025, Dapeng Mi wrote:
> > >> BTW, Sean, may I know your plan about the mediated vPMU v5 patch set? Thanks.
> > > I'll get it out this week (hopefully tomorrow).
> > 
> > Thumbs up! Thanks.
> 
> I lied, I'm not going to get it out until Monday.  Figuring out how to deal with
> instruction emulation in the fastpath VM-Exit handlers took me longer than I was
> hoping/expecting.
> 
> It's fully tested, and I have all but one changelog written, but I'm out of time
> for today (I made a stupid goof (inverted a !) that cost me an ~hour today, *sigh*).
> 
> Unless I get hit by a meteor, I'll get it out Monday.

*sigh*

Wrong again (fortunately, I didn't get hit by a meteor).  Long story short, I
revisited (yet again) how to deal with enabling the mediated PMU.  I had been
doing almost all of my testing with a hack to force-enable a mediated PMU, and
when it came time to rip that out, I just couldn't convince myself that requiring
userspace to enable KVM_CAP_PMU_CAPABILITY was the best behavior (I especially
hated that PMU support would silently disappear).

So, bad news is, v5 isn't happening today.  Good news is that I think I figured
out a not-awful solution for enabling the mediated PMU.  I'll post details (and
hopefully v5) tomorrow.