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Date:   Thu, 3 May 2018 16:38:23 +0300
From:   Alexander Shishkin <alexander.shishkin@linux.intel.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Luwei Kang <luwei.kang@intel.com>, kvm@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
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        joro@8bytes.org, peterz@infradead.org, chao.p.peng@linux.intel.com
Subject: Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace
 virtualization mode
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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.

Regards,
--
Alex