Messages in this thread | | | Date | Thu, 3 May 2018 16:38:23 +0300 | From | Alexander Shishkin <> | 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
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