[PATCH v7 01/10] KVM: arm64: Allow userspace to get the writable masks for feature ID registers

[Jing Zhang]

On Wed, Aug 2, 2023 at 10:04 AM Oliver Upton <oliver.upton at linux.dev> wrote:
>
> On Wed, Aug 02, 2023 at 08:55:43AM -0700, Jing Zhang wrote:
> > > > +#define ARM64_FEATURE_ID_SPACE_SIZE  (3 * 8 * 8)
> > > > +
> > > > +struct feature_id_writable_masks {
> > > > +     __u64 mask[ARM64_FEATURE_ID_SPACE_SIZE];
> > > > +};
> > >
> > > This UAPI is rather difficult to extend in the future. We may need to
> > > support describing the masks of multiple ranges of registers in the
> > > future. I was thinking something along the lines of:
> > >
> > >         enum reg_mask_range_idx {
> > >                 FEATURE_ID,
> > >         };
> > >
> > >         struct reg_mask_range {
> > >                 __u64 idx;
> > >                 __u64 *masks;
> > >                 __u64 rsvd[6];
> > >         };
> > >
> > Since have the way to map sysregs encoding to the index in the mask
> > array, we can extend the UAPI by just adding a size field in struct
> > feature_id_writable_masks like below:
> > struct feature_id_writable_masks {
> >          __u64 size;
> >          __u64 mask[ARM64_FEATURE_ID_SPACE_SIZE];
> > };
> > The 'size' field can be used as input for the size of 'mask' array and
> > output for the number of masks actually read in.
> > This way, we can freely add more ranges without breaking anything in userspace.
> > WDYT?
>
> Sorry, 'index' is a bit overloaded in this context. The point I was
> trying to get across is that we might want to describe a completely
> different range of registers than the feature ID registers in the
> future. Nonetheless, we shouldn't even presume the shape of future
> extensions to the ioctl.
>
>         struct reg_mask_range {
>                 __u64 addr;     /* pointer to mask array */
>                 __u64 rsvd[7];
>         };
>
> Then in KVM we should require ::rsvd be zero and fail the ioctl
> otherwise.
Got it. Will add the ::rsvd for future expansion.

Thanks,
Jing
>
> --
> Thanks,
> Oliver