[PATCH v4 1/4] KVM: arm64: Enable writable for ID_AA64DFR0_EL1

[Oliver Upton]

Hi Cornelia,

On Tue, Jul 04, 2023 at 05:06:30PM +0200, Cornelia Huck wrote:
> On Mon, Jun 26 2023, Oliver Upton <oliver.upton at linux.dev> wrote:
> 
> > On Wed, Jun 07, 2023 at 07:45:51PM +0000, Jing Zhang wrote:
> >> +	brps = FIELD_GET(ID_AA64DFR0_EL1_BRPs_MASK, val);
> >> +	ctx_cmps = FIELD_GET(ID_AA64DFR0_EL1_CTX_CMPs_MASK, val);
> >> +	if (ctx_cmps > brps)
> >> +		return -EINVAL;
> >> +
> >
> > I'm not fully convinced on the need to do this sort of cross-field
> > validation... I think it is probably more trouble than it is worth. If
> > userspace writes something illogical to the register, oh well. All we
> > should care about is that the advertised feature set is a subset of
> > what's supported by the host.
> >
> > The series doesn't even do complete sanity checking, and instead works
> > on a few cherry-picked examples. AA64PFR0.EL{0-3} would also require
> > special handling depending on how pedantic you're feeling. AArch32
> > support at a higher exception level implies AArch32 support at all lower
> > exception levels.
> >
> > But that isn't a suggestion to implement it, more of a suggestion to
> > just avoid the problem as a whole.
> 
> Generally speaking, how much effort do we want to invest to prevent
> userspace from doing dumb things? "Make sure we advertise a subset of
> features of what the host supports" and "disallow writing values that
> are not allowed by the architecture in the first place" seem reasonable,
> but if userspace wants to create weird frankencpus[1], should it be
> allowed to break the guest and get to keep the pieces?

What I'm specifically objecting to is having KVM do sanity checks across
multiple fields. That requires explicit, per-field plumbing that will
eventually become a tangled mess that Marc and I will have to maintain.
The context-aware breakpoints is one example, as is ensuring SVE is
exposed iff FP is too. In all likelihood we'll either get some part of
this wrong, or miss a required check altogether.

Modulo a few exceptions to this case, I think per-field validation is
going to cover almost everything we're worried about, and we get that
largely for free from arm64_check_features().

> I'd be more in favour to rely on userspace to configure something that
> is actually usable; it needs to sanitize any user-provided configuration
> anyway.

Just want to make sure I understand your sentiment here, you'd be in
favor of the more robust sanitization?

> [1] I think userspace will end up creating frankencpus in any case, but
> at least it should be the kind that doesn't look out of place in the
> subway if you dress it in proper clothing.

I mean, KVM already advertises a frankencpu in the first place, so we're
off to a good start :)

--
Thanks,
Oliver