Re: [PATCH v6 3/6] KVM: arm64: Enable writable for ID_AA64DFR0_EL1 and ID_DFR0_EL1

[Oliver Upton <oliver.upton@linux.dev>]

Hi Cornelia,

On Mon, Jul 24, 2023 at 10:45:44AM +0200, Cornelia Huck wrote:
> On Fri, Jul 21 2023, Oliver Upton <oliver.upton@linux.dev> wrote:
> > What I had in mind was something similar to the KVM_GET_ONE_REG ioctl,
> > but instead of returning the register value it'd return the mask of the
> > register. This would keep the kernel implementation dead simple (I'm
> > lazy) and more easily allow for future expansion in case we want to
> > start describing more registers this way. Userspace would iterate the ID
> > register space and ask the kernel for the mask of registers it wants to
> > change.
> 
> Hm... for userspace it might be easier to get one big list and then
> parse it afterwards? Similar to what GET_REG_LIST does today.

Possibly, but I felt like it was a bit different from GET_REG_LIST since
this would actually be a list of key-value pairs (reg_id, mask) instead
of a pure enumeration of IDs. My worry is that if/when we wind up describing
more registers in this list-based ioctl then userspace is going to wind
up traversing that structure a lot to find the register masks it actually
cares about.

> Are you thinking more of a KVM_GET_REG_INFO or so ioctl, that could
> support different kinds of extra info (and might also make sense for
> other architectures?) If we end up with something more versatile, it
> might make sense going that route.

TBH, I hadn't considered the extensibililty of a per-register ioctl, but
that does seem like a good point.

--
Thanks,
Oliver