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From:   Alexander Shishkin <alexander.shishkin@linux.intel.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Luwei Kang <luwei.kang@intel.com>, kvm@vger.kernel.org,
        x86@kernel.org
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        linux-kernel@vger.kernel.org,
        Chao Peng <chao.p.peng@linux.intel.com>
Subject: Re: [PATCH v13 08/12] KVM: x86: Add Intel PT context switch for each vcpu
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Date:   Wed, 31 Oct 2018 13:38:39 +0200
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Paolo Bonzini <pbonzini@redhat.com> writes:

> On 30/10/2018 12:26, Alexander Shishkin wrote:
>> There is a control in the perf event attribute that enables tracing the
>> guest. If this control is enabled, the kvm needs to stay away from any
>> PT related MSRs.
>
> This cannot happen once the guest has been told it can trace itself.

So, they need to be made mutually exclusive.

> There is no standard way to tell the guest that the host overrode its
> choice to use PT.  However, the host will get a PGD/PGE packet around
> vmentry and vmexit, so there _will_ be an indication that the guest
> owned the MSRs for that period of time.

Not if they are not tracing the kernel.

> If PT context switching is enabled with the module parameter, we could
> also reject creation of events with the attribute set.  However that
> won't help if the event is created before KVM is even loaded.

In that case, modprobe kvm should fail.

Regards,
--
Alex