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Date:   Wed, 9 Feb 2022 21:00:08 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Like Xu <like.xu.linux@gmail.com>,
        Jim Mattson <jmattson@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Stephane Eranian <eranian@google.com>,
        David Dunn <daviddunn@google.com>
Subject: Re: KVM: x86: Reconsider the current approach of vPMU
Message-ID: <YgQrWHCNG/s4EWFf@google.com>
References: <20220117085307.93030-1-likexu@tencent.com>
 <20220117085307.93030-3-likexu@tencent.com>
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On Wed, Feb 09, 2022, Peter Zijlstra wrote:
> 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.

The proposal is to add an option to allow userspace to gift the PMU to the guest,
not to let the guest steal the PMU at will.  Off by default, certain capabilities
required, etc... are all completely ok and desired, e.g. we also have use cases
where we don't want to let the guest touch the PMU.

David's response in the original thread[*] explains things far better than I can do.

[*] https://lore.kernel.org/all/CABOYuvbPL0DeEgV4gsC+v786xfBAo3T6+7XQr7cVVzbaoFoEAg@mail.gmail.com