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Subject: Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace
 virtualization mode
To:     Peter Zijlstra <peterz@infradead.org>
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,
        x86@kernel.org, rkrcmar@redhat.com, linux-kernel@vger.kernel.org,
        joro@8bytes.org, chao.p.peng@linux.intel.com
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From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <6b6a82f9-4bb5-b072-b829-a0903d9366fa@redhat.com>
Date:   Mon, 7 May 2018 12:47:41 +0200
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On 05/05/2018 00:15, Peter Zijlstra wrote:
> On Fri, May 04, 2018 at 11:44:09PM +0200, Paolo Bonzini wrote:
>> On 04/05/2018 12:45, Peter Zijlstra wrote:
>>> On Thu, May 03, 2018 at 04:38:23PM +0300, Alexander Shishkin wrote:
>>>> On Thu, May 03, 2018 at 02:50:12PM +0200, Paolo Bonzini wrote:
>>>
>>>>> 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.
>>>
>>> Yes, that sounds like a much better approach.
>>
>> I don't think so.  The possibility that the host would lose tracing data
>> just because the guest starts using PT seems hideous to me...
> 
> Well, either way around is a fairly crap situation, the modparam at
> least lets the admin pick which it goes. But if you want to always let
> the host win, that's fine with me too, less knobs is better.

I expect that the default "system-wide" host wins will be used almost
always, with "host-guest" being used in case someone actually wants to
use PT in guests.

I agree that "Host-only, drop guest events" should be removed, since it
can be emulated by perf code.  Luwei, can you change that?

Paolo