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Date:   Thu, 3 May 2018 15:48:47 +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,
        x86@kernel.org, rkrcmar@redhat.com, linux-kernel@vger.kernel.org,
        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 02:30:25PM +0200, Paolo Bonzini wrote:
> On 03/05/2018 14:02, Alexander Shishkin wrote:
> > I don't see why. System-wide tracing takes place while perf record is
> > running. When it's done, it's done and we can un-disable the guest
> > tracing, without requiring the user to kill all their VMs and reload
> > modules.
> 
> 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.

Side note: the "system-wide tracing" is a misnomer here, all that matters
is that there's a perf event on the host that wants to trace the guest, it
can very well be a per-task event.

Regards,
--
Alex