On 2022-09-27 10:27 a.m., Wang, Wei W wrote: > On Tuesday, September 27, 2022 2:09 AM, Liang, Kan wrote: >> From my understanding of the host-guest mode, the host PT event never >> traces the guest no matter whether the guest enables PT. So when VM-entry, >> there is only a guest PT event or no event. If so, I think the perf tool should >> warn the user if they try to create a host event with !exclude_guest, since the >> host event never traces a guest. > > Probably not from the perf side. It's actually an issue of the KVM's pt_mode: > How can KVM prevent host from profiling the guest when in host-guest mode? I don't think it's KVM's job to prevent host from profiling the guest. However, the current KVM implementation implicitly disables the guest profiling from the host. If I understand correct, this patch series just change it to an explicit way (Just for kernel. Perf tool still doesn't know about it.). There is no substantial change. I think it should be PT driver who decide which event should be scheduled. So the first step is to let the PT driver understand the host-guest mode. Then if a perf user in the host tries to profile with !exclude_guest with the host-guest mode, the pt driver can deny the request. The perf tool than throw a warning, e.g., "The VMM is running with host-guest mode. Perf cannot profile guest. Please remove the guest profiling setting or switch to the host-only mode.". > Just a warning from the perf side wouldn't be enough. I think that's a wrong > assumption from KVM side. I had a plan to fix this one. Exposing the host event > can tell KVM if the host is profiling the guest or not (i.e. host-guest is allowed). I don't think KVM should know such information. Thanks, Kan
