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

[Jing Zhang]

Hi Oliver, Cornelia,

Thanks for the discussion about the cross-field validation. I'm happy
to know that we all agree to avoid that. I'll remove those validations
for later posts.

Thanks,
Jing

On Wed, Jul 5, 2023 at 1:49 AM Cornelia Huck <cohuck at redhat.com> wrote:
>
> On Tue, Jul 04 2023, Oliver Upton <oliver.upton at linux.dev> wrote:
>
> > 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.
>
> Nod, this sounds like more trouble than it's worth in the end.
>
> >
> > 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?
>
> In userspace. E.g. QEMU can go ahead and try to implement the
> user-exposed knobs in a way that the really broken configurations are
> not even possible. I'd also expect userspace to have a more complete
> view of what it is trying to instantiate (especially if code is shared
> between instantiating a vcpu for use with KVM and a fully emulated
> vcpu -- we probably don't want to go all crazy in the latter case,
> either.)
>
> >
> >> [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 :)
>
> Indeed :)
>