Re: [PATCH v6 06/14] KVM: Add memslot flag to let userspace force an exit on missing hva mappings

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 31, 2024, Anish Moorthy wrote:
> On Tue, Jan 30, 2024 at 4:26 PM James Houghton <jthoughton@google.com> wrote:
> >
> > Feel free to add:
> >
> > Reviewed-by: James Houghton <jthoughton@google.com>
> 
> > If we include KVM_MEM_GUEST_MEMFD here, we should point the reader to
> > KVM_SET_USER_MEMORY_REGION2 and explain that using
> > KVM_SET_USER_MEMORY_REGION with this flag will always fail.
> 
> Done and done (I've split the guest memfd doc update off into its own
> commit too).
> 
> > > @@ -3070,6 +3074,15 @@ kvm_pfn_t __gfn_to_pfn_memslot(const struct kvm_memory_slot *slot, gfn_t gfn,
> > >                 writable = NULL;
> > >         }
> > >
> > > +       if (!atomic && can_exit_on_missing
> > > +           && kvm_is_slot_exit_on_missing(slot)) {

Operators go on the preceding line:

	if (!atomic && can_exit_on_missing &&
	    kvm_is_slot_exit_on_missing(slot))

> > > +               atomic = true;
> > > +               if (async) {
> > > +                       *async = false;
> > > +                       async = NULL;
> > > +               }
> > > +       }
> > > +
> >
> > Perhaps we should have a comment for this? Maybe something like: "If
> > we want to exit-on-missing, we want to bail out if fast GUP fails, and
> > we do not want to go into slow GUP. Setting atomic=true does exactly
> > this."
> 
> I was going to push back on the use of "we" but I see that it's all
> over kvm_main.c :).

Ignore the ancient art, push back.  The KVM code base is 15 years old at this
point.  A _lot_ of has changed in 15 years.  The fact that KVM has existing code
that's in violation of current standards doesn't justify ignoring the standards
when adding new code.