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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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Message-ID: <e948d77e-b96e-147a-745d-aad05935b61b@redhat.com>
Date:   Thu, 3 May 2018 15:48:12 +0200
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On 03/05/2018 15:38, Alexander Shishkin wrote:
> On Thu, May 03, 2018 at 02:50:12PM +0200, Paolo Bonzini wrote:
>> On 03/05/2018 14:48, Alexander Shishkin wrote:
>>>> Guest tracing can only be enabled at boot time, because the guest's
>>>> CPUID changes depending on whether it's enabled.  And likewise if perf
>>>> record can do system-wide tracing at any time during the guest's
>>>> execution, we need to know it at boot time in order to set the guest CPUID.
>>>
>>> CPUID is immaterial here; the real trick is to disallow the use of PT at
>>> runtime when the host suddenly decides to trace the guest, in such a way
>>> that the guest user is informed that their trace is incomplete due to the
>>> host activity.
>>
>> How do you do that?
> 
> Off the top of my head:
>   * you don't;
>   * you write something to the PT stream;
>   * you signal an error via RTIT_STATUS;
>   * guest always prevails: host gets PARTIAL records in case of a conflict.
> 
>> And you still need the module parameter to decide
>> whether the host is _allowed_ to cause incomplete traces in the guest.
> 
> 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.

Paolo