Re: [PATCH 4/4] KVM: arm64: Refuse to run VCPU if the PMU doesn't match the physical CPU

[Alexandru Elisei <alexandru.elisei@arm.com>]

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@arm.com> 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@arm.com> 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@arm.com> 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.
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