Re: [PATCH] KVM: x86/svm: Add module param to control PMU virtualization

[Jim Mattson]

On Fri, Dec 10, 2021 at 7:48 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> On Fri, Dec 10, 2021 at 6:15 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> >
> > On 12/10/21 20:25, Jim Mattson wrote:
> > > In the long run, I'd like to be able to override this system-wide
> > > setting on a per-VM basis, for VMs that I trust. (Of course, this
> > > implies that I trust the userspace process as well.)
> > >
> > > How would you feel if we were to add a kvm ioctl to override this
> > > setting, for a particular VM, guarded by an appropriate permissions
> > > check, like capable(CAP_SYS_ADMIN) or capable(CAP_SYS_MODULE)?
> >
> > What's the rationale for guarding this with a capability check?  IIRC
> > you don't have such checks for perf_event_open (apart for getting kernel
> > addresses, which is not a problem for virtualization).
>
> My reasoning was simply that for userspace to override a mode 0444
> kernel module parameter, it should have the rights to reload the
> module with the parameter override. I wasn't thinking specifically
> about PMU capabilities.

Assuming that we trust userspace to decide whether or not to expose a
virtual PMU to a guest (as we do on the Intel side), perhaps we could
make use of the existing PMU_EVENT_FILTER to give us per-VM control,
rather than adding a new module parameter for per-host control. If
userspace calls KVM_SET_PMU_EVENT_FILTER with an action of
KVM_PMU_EVENT_ALLOW and an empty list of allowed events, KVM could
just disable the virtual PMU for that VM.

Today, the semantics of an empty allow list are quite different from
the proposed pmuv module parameter being false. However, it should be
an easy conversion. Would anyone be concerned about changing the
current semantics of an empty allow list? Is there a need for
disabling PMU virtualization for legacy userspace implementations that
can't be modified to ask for an empty allow list?