On Fri, Mar 06, 2020 at 09:42:47AM -0500, Liang, Kan wrote: > > > On 3/6/2020 8:53 AM, Peter Zijlstra wrote: > > On Fri, Mar 06, 2020 at 01:56:55AM +0800, Luwei Kang wrote: > > > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx> > > > > > > The PEBS event created by host needs to be assigned specific counters > > > requested by the guest, which means the guest and host counter indexes > > > have to be the same or fail to create. This is needed because PEBS leaks > > > counter indexes into the guest. Otherwise, the guest driver will be > > > confused by the counter indexes in the status field of the PEBS record. > > > > > > A guest_dedicated_idx field is added to indicate the counter index > > > specifically requested by KVM. The dedicated event constraints would > > > constrain the counter in the host to the same numbered counter in guest. > > > > > > A intel_ctrl_guest_dedicated_mask field is added to indicate the enabled > > > counters for guest PEBS events. The IA32_PEBS_ENABLE MSR will be switched > > > during the VMX transitions if intel_ctrl_guest_owned is set. > > > > > > > > + /* the guest specified counter index of KVM owned event, e.g PEBS */ > > > + int guest_dedicated_idx; > > > > We've always objected to guest 'owned' counters, they destroy scheduling > > freedom. Why are you expecting that to be any different this time? > > > > The new proposal tries to 'own' a counter by setting the event constraint. > It doesn't stop other events using the counter. > If there is high priority event which requires the same counter, scheduler > can still reject the request from KVM. > I don't think it destroys the scheduling freedom this time. Suppose your KVM thing claims counter 0/2 (ICL/SKL) for some random PEBS event, and then the host wants to use PREC_DIST.. Then one of them will be screwed for no reason what so ever. How is that not destroying scheduling freedom? Any other situation we'd have moved the !PREC_DIST PEBS event to another counter.
