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.
