linux-kernel - Re: [PATCH v5 09/44] perf/x86: Switch LVTPC to/from mediated PMI vector on guest load/put context

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Message-ID: <20250818161210.GJ3289052@noisy.programming.kicks-ass.net>
Date: Mon, 18 Aug 2025 18:12:10 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
	Tianrui Zhao <zhaotianrui@...ngson.cn>,
	Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>,
	Anup Patel <anup@...infault.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Xin Li <xin@...or.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	kvm@...r.kernel.org, loongarch@...ts.linux.dev,
	kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
	Kan Liang <kan.liang@...ux.intel.com>,
	Yongwei Ma <yongwei.ma@...el.com>,
	Mingwei Zhang <mizhang@...gle.com>,
	Xiong Zhang <xiong.y.zhang@...ux.intel.com>,
	Sandipan Das <sandipan.das@....com>,
	Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [PATCH v5 09/44] perf/x86: Switch LVTPC to/from mediated PMI
 vector on guest load/put context

On Mon, Aug 18, 2025 at 08:25:34AM -0700, Sean Christopherson wrote:

> > OK, so *IF* doing the VM-exit during PMI is sound, this is something
> > that needs a comment somewhere. 
> 
> I'm a bit lost here.  Are you essentially asking if it's ok to take a VM-Exit
> while the guest is handling a PMI?  If so, that _has_ to work, because there are
> myriad things that can/will trigger a VM-Exit at any point while the guest is
> active.

Yes, that's what I'm asking. Why is this VM-exit during PMI nonsense not
subject to the same failures that mandates the mid/late PMI ACK.

And yes, I realize this needs to work. But so far I'm not sure I
understand why that is a safe thing to do.

Like I wrote, I suspect writing all the PMU MSRs serializes things
sufficiently, but if that is the case, that needs to be explicitly
mentioned. Because that also doesn't explain why we needs mid-ack
instead of late-ack on ADL e-cores for instance.

Could it perhaps be that we don't let the guests do PEBS because DS
doesn't virtualize? And thus we don't have the malformed PEBS record?