Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace virtualization mode

[Paolo Bonzini]

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?  And you still need the module parameter to decide
whether the host is _allowed_ to cause incomplete traces in the guest.

Paolo

> 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.