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

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

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.

-- 
Thanks,
Oliver

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