linux-kernel - Re: [PATCH kvm/queue v2 2/3] perf: x86/core: Add interface to query perfmon_event

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Date:   Wed, 9 Feb 2022 10:47:27 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Like Xu <like.xu.linux@...il.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, Like Xu <likexu@...cent.com>,
        Stephane Eranian <eranian@...gle.com>,
        David Dunn <daviddunn@...gle.com>
Subject: Re: [PATCH kvm/queue v2 2/3] perf: x86/core: Add interface to query
 perfmon_event_map[] directly

On Wed, Feb 9, 2022 at 7:41 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 2/9/22 05:21, Peter Zijlstra wrote:
> > On Wed, Feb 02, 2022 at 02:35:45PM -0800, 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
> >> subsystem what's happening, or other host perf clients are going to
> >> get bogus data.
> > That's not acceptible behaviour. I'm all for unilaterally killing any
> > guest that does this.
>
> I'm not sure where the "bogus data" comes or to what that refers
> specifically.  But, the host does have some level of control:

I was referring to gaps in the collection of data that the host perf
subsystem doesn't know about if ATTRIBUTES.PERFMON is set for a TDX
guest. This can potentially be a problem if someone is trying to
measure events per unit of time.

> > The host VMM controls whether a guest TD can use the performance
> > monitoring ISA using the TD’s ATTRIBUTES.PERFMON bit...
>
> So, worst-case, we don't need to threaten to kill guests.  The host can
> just deny access in the first place.
>
> I'm not too picky about what the PMU does, but the TDX behavior didn't
> seem *that* onerous to me.  The gory details are all in "On-TD
> Performance Monitoring" here:
>
> > https://www.intel.com/content/dam/develop/external/us/en/documents/tdx-module-1.0-public-spec-v0.931.pdf
>
> My read on it is that TDX host _can_ cede the PMU to TDX guests if it
> wants.  I assume the context-switching model Jim mentioned is along the
> lines of what TDX is already doing on host<->guest transitions.

Right. If ATTRIBUTES.PERFMON is set, then "perfmon state is
context-switched by the Intel TDX module across TD entry and exit
transitions." Furthermore, the VMM has no access to guest perfmon
state.

If you're saying that setting this bit is unacceptable, then perhaps
the TDX folks need to redesign their in-guest PMU support.