[PATCH v8 20/20] KVM: ARM64: Add a new kvm ARM PMU device

[Andrew Jones]

On Mon, Jan 11, 2016 at 04:29:03PM +0000, Peter Maydell wrote:
> On 11 January 2016 at 16:21, Andrew Jones <drjones at redhat.com> wrote:
> > On Mon, Jan 11, 2016 at 05:09:27PM +0100, Andrew Jones wrote:
> >> On Mon, Jan 11, 2016 at 04:09:29PM +0100, Christoffer Dall wrote:
> >> > Are vcpu ids already exposed to userspace (beyond the stupid
> >> > KVM_IRQ_LINE) ioctl and as such we're bound to whatever upper limit and
> >> > format they have?
> >>
> >> The only other place I found is KVM_CREATE_VCPU. I suppose we could move
> >> to MPIDR for that, and it would be a nice way to handle the "userspace
> >> determines MPIDR" work that I plan to do. Both KVM and its userspaces
> >> would still use some counter-based vcpu identifiers internally, to avoid
> >> large, sparse structures, but I guess the advantage is that they don't
> >> have to agree on how they do that. The 'vcpu id' used by KVM_CREATE_VCPU
> >> is already 32-bits, and is supposed to be an arbitrary identifier. That
> >> all looks good for converting to MPIDR.
> >>
> >
> > Correction. I understand that vcpu-id is "supposed" to be an arbitrary
> > identifier now, but it doesn't appear that all the assumptions that it's
> > a counter are gone yet... virt/kvm/kvm_main.c has
> >
> > static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
> > ...
> >         if (id >= KVM_MAX_VCPUS)
> >                 return -EINVAL;
> 
> I think the last time we talked about supporting "userspace
> determines MPIDR" the idea was to do it by allowing userspace to
> write to the MPIDR register with KVM_SET_ONE_REG. So you'd
> create a bunch of CPUs with vcpu-ids as usual, and then the
> MPIDRs would be set for them later as appropriate (or not
> at all, if userspace was an older qemu).

Yup, I recall that. I'm just expanding this discussion into that one.
If we wanted to single vcpu identifier type, and we wanted it to be
MPIDR, then I guess we'd want to pass it in to KVM_CREATE_VCPU too, at
which point we no longer need to set it later with KVM_SET_ONE_REG.

Thanks,
drew