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
