On 03/05/2018 15:38, Alexander Shishkin wrote: > 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. It's not different versions; it's having PT vs. not having PT at all. I don't really see it as a big issue. The nice thing about this series is that the interactions between PT code and KVM code are minimal. Paolo
