linux-kernel - Re: KVM: x86: Reconsider the current approach of vPMU

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Message-ID: <YgPCm1WIt9dHuoEo@hirez.programming.kicks-ass.net>
Date:   Wed, 9 Feb 2022 14:33:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>,
        David Dunn <daviddunn@...gle.com>
Subject: Re: KVM: x86: Reconsider the current approach of vPMU

On Wed, Feb 09, 2022 at 04:10:48PM +0800, Like Xu wrote:
> On 3/2/2022 6:35 am, Jim Mattson wrote:
> > 3) TDX is going to pull the rug out from under us anyway. When the TDX
> > module usurps control of the PMU, any active host counters are going
> > to stop counting. We are going to need a way of telling the host perf
> 
> I presume that performance counters data of TDX guest is isolated for host,
> and host counters (from host perf agent) will not stop and keep counting
> only for TDX guests in debug mode.

Right, lots of people like profiling guests from the host. That allows
including all the other virt gunk that supports the guest.

Guests must not unilaterally steal the PMU.

> At one time, we proposed to statically reserve counters from the host
> perf view at guest startup, but this option was NAK-ed from PeterZ.

Because counter constraints, if you hand C0 to the guest, the host
can no longer count certain events, which is bad.