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Date:   Mon, 9 Mar 2020 08:44:45 -0700
From:   Andi Kleen <ak@linux.intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        Luwei Kang <luwei.kang@intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        mingo@redhat.com, acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org, tglx@linutronix.de, bp@alien8.de,
        hpa@zytor.com, pbonzini@redhat.com,
        sean.j.christopherson@intel.com, vkuznets@redhat.com,
        wanpengli@tencent.com, jmattson@google.com, joro@8bytes.org,
        pawan.kumar.gupta@linux.intel.com, thomas.lendacky@amd.com,
        fenghua.yu@intel.com, like.xu@linux.intel.com
Subject: Re: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a
 dedicated counter for guest PEBS
Message-ID: <20200309154445.GL1454533@tassilo.jf.intel.com>
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> Suppose your KVM thing claims counter 0/2 (ICL/SKL) for some random PEBS
> event, and then the host wants to use PREC_DIST.. Then one of them will
> be screwed for no reason what so ever.

It's no different from some user using an event that requires some
specific counter.

> 
> How is that not destroying scheduling freedom? Any other situation we'd
> have moved the !PREC_DIST PEBS event to another counter.

Anyways what are you suggesting to do instead? Do you have a better proposal?

The only alternative I know to doing this would be to go through the PEBS
buffer in the guest and patch the applicable counter field up on each PMI.

I tried that at some point (still have code somewhere), but it was
quite complicated and tricky and somewhat slow, so I gave up eventually.

It's also inherently racy because if the guest starts looking at
the PEBS buffer before an PMI it could see the unpatched values
Given I don't know any real software which would break from this,
but such "polled PEBS" usages are certainly concievable.

The artificial constraint is a lot simpler and straight forward,
and also doesn't have any holes like this.

-Andi