On Thu, Nov 17, 2022 at 06:23:23PM +0000, Oliver Upton wrote:
> On Thu, Nov 17, 2022 at 05:49:52PM +0000, Will Deacon wrote:
> > On Wed, Nov 16, 2022 at 04:56:55PM +0000, Oliver Upton wrote:
>
> [...]
>
> > > -static inline void kvm_pgtable_walk_begin(void) {}
> > > -static inline void kvm_pgtable_walk_end(void) {}
> > > +static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
> > > +{
> > > + /*
> > > + * Due to the lack of RCU (or a similar protection scheme), only
> > > + * non-shared table walkers are allowed in the hypervisor.
> > > + */
> > > + WARN_ON(walker->flags & KVM_PGTABLE_WALK_SHARED);
> > > +}
> >
> > I think it would be better to propagate the error to the caller rather
> > than WARN here.
>
> I'd really like to warn somewhere though since we're rather fscked at
> this point. Keeping that WARN close to the exceptional condition would
> help w/ debugging.
>
> Were you envisioning bubbling the error all the way back up (i.e. early
> return from kvm_pgtable_walk())?
Yes, that's what I had in mind. WARN is fatal at EL2, so I think it's
better to fail the pgtable operation rather than bring down the entire
machine by default. Now, it _might_ be fatal anyway (e.g. if we were
handling a host stage-2 fault w/ pKVM), but the caller is in a better
position to decide the severity.
> I had really only intended these to indirect lock acquisition/release,
> so the error handling on the caller side feels weird:
>
> static inline int kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
> {
> if (WARN_ON(walker->flags & KVM_PGTABLE_WALK_SHARED))
> return -EPERM;
>
> return 0;
> }
>
> r = kvm_pgtable_walk_begin()
> if (r)
> return r;
>
> r = _kvm_pgtable_walk();
> kvm_pgtable_walk_end();
This doesn't look particularly weird to me (modulo dropping the WARN, or
moving it to _end()), but maybe I've lost my sense of taste.
> > Since you're rejigging things anyway, can you have this
> > function return int?
>
> If having this is a strong motivator I can do a v4.
It's a really minor point, so I'll leave it up to you guys.
WIll
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