On Mon, 22 Nov 2021 12:12:17 +0000, Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote: > > Hi Marc, > > On Sun, Nov 21, 2021 at 07:35:13PM +0000, Marc Zyngier wrote: > > On Mon, 15 Nov 2021 16:50:41 +0000, > > Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote: > > > > > > Userspace can assign a PMU to a VCPU with the KVM_ARM_VCPU_PMU_V3_SET_PMU > > > device ioctl. If the VCPU is scheduled on a physical CPU which has a > > > different PMU, the perf events needed to emulate a guest PMU won't be > > > scheduled in and the guest performance counters will stop counting. Treat > > > it as an userspace error and refuse to run the VCPU in this situation. > > > > > > The VCPU is flagged as being scheduled on the wrong CPU in vcpu_load(), but > > > the flag is cleared when the KVM_RUN enters the non-preemptible section > > > instead of in vcpu_put(); this has been done on purpose so the error > > > condition is communicated as soon as possible to userspace, otherwise > > > vcpu_load() on the wrong CPU followed by a vcpu_put() could clear the flag. > > > > Can we make this something orthogonal to the PMU, and get userspace to > > pick an affinity mask independently of instantiating a PMU? I can > > imagine this would also be useful for SPE on asymmetric systems. > > I actually went this way for the latest version of the SPE series [1] and > dropped the explicit userspace ioctl in favor of this mechanism. > > The expectation is that userspace already knows which CPUs are associated > with the chosen PMU (or SPE) when setting the PMU for the VCPU, and having > userspace set it explicitely via an ioctl looks like an unnecessary step to > me. I don't see other usecases of an explicit ioctl outside of the above > two situation (if userspace wants a VCPU to run only on specific CPUs, it > can use thread affinity for that), so I decided to drop it. My problem with that is that if you have (for whatever reason) a set of affinities that are not strictly identical for both PMU and SPE, and expose both of these to a guest, what do you choose? As long as you have a single affinity set to take care of, you're good. It is when you have several ones that it becomes ugly (as with anything involving asymmetric CPUs). M. -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
