Hi Marc, On Mon, Dec 06, 2021 at 10:15:31AM +0000, Marc Zyngier wrote: > On Mon, 22 Nov 2021 14:43:17 +0000, > Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote: > > > > Hi Marc, > > > > On Mon, Nov 22, 2021 at 02:21:00PM +0000, Marc Zyngier wrote: > > > 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). > > > > I thought about it when I decided to do it this way, my solution was to do > > a cpumask_and() with the existing VCPU cpumask when setting a VCPU feature > > that requires it, and returning an error if we get an empty cpumask, > > because userspace is requesting a combination of VCPU features that is not > > supported by the hardware. > > So every new asymetric feature would come with its own potential > affinity mask, and KVM would track the restriction of that affinity. I > guess that because it can only converge to zero, this is safe by > design... > > One thing I want to make sure is that we can evaluate the mask very > early on, and reduce the overhead of that evaluation. I don't think the check can be made any sooner than when the feature bit is set, which is what I am proposing :) > > > Going with the other solution (user sets the cpumask via an ioctl), KVM > > would still have to check against certain combinations of VCPU features > > (for SPE it's mandatory, so KVM doesn't trigger an undefined exception, we > > could skip the check for PMU, but then what do we gain from the ioctl if > > KVM doesn't check that it matches the PMU?), so I don't think we loose > > anything by going with the implicit cpumask. > > > > What do you think? > > OK, fair enough. Please respin the series (I had a bunch of minor > comments), and I'll have another look. Great, thanks! Alex > > Thanks, > > M. > > -- > Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
