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Subject: Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace
 virtualization mode
To:     Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc:     Luwei Kang <luwei.kang@intel.com>, kvm@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
        x86@kernel.org, rkrcmar@redhat.com, linux-kernel@vger.kernel.org,
        joro@8bytes.org, peterz@infradead.org, chao.p.peng@linux.intel.com
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From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Fri, 4 May 2018 23:52:48 +0200
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On 04/05/2018 12:38, Alexander Shishkin wrote:
>>> Or rather a parameter to decide who wins in case both host and guest want
>>> to trace the guest. That's arguably better than having different versions of
>>> PT in the guest depending on a module parameter setting.
>> It's not different versions; it's having PT vs. not having PT at all.  I
>> don't really see it as a big issue.  The nice thing about this series is
>> that the interactions between PT code and KVM code are minimal.
> Unfortunately, it gets it wrong. Like I just said in another email, if you
> switch off host's PT, you need to let them know, which this patchset doesn't
> do. And when it does, it would be the same amount of interaction with PT
> code as what would be required to get the dynamic guest PT right.

Two issues:

1) Is there a fast (10 clock cycles, better if less) way for KVM to know
"PT is enabled on the host", or a callback that KVM can register when
e.g. RTIT_CTL is written?

2) We'd have to write trace records into the guest.  That does not sound
that easy.  Does it entail parsing the ToPA and all that?

Thanks,

Paolo