Re: [PATCH v2] KVM: x86: Add dedicated helper to get CPUID entry with significant index

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Tue, Jul 12, 2022, Sean Christopherson wrote:
> On Tue, Jul 12, 2022, Maxim Levitsky wrote:
> > On Tue, 2022-07-12 at 00:06 +0000, Sean Christopherson wrote:
> > > +               /*
> > > +                * Function matches and index is significant; not specifying an
> > > +                * exact index in this case is a KVM bug.
> > > +                */
> > Nitpick: Why KVM bug? Bad userspace can also provide a index-significant entry for cpuid
> > leaf for which index is not significant in the x86 spec.
> 
> Ugh, you're right.
> 
> > We could arrange a table of all known leaves and for each leaf if it has an index
> > in the x86 spec, and warn/reject the userspace CPUID info if it doesn't match.
> 
> We have such a table, cpuid_function_is_indexed().  The alternative would be to
> do:
> 
> 		WARN_ON_ONCE(index == KVM_CPUID_INDEX_NOT_SIGNIFICANT &&
> 			     cpuid_function_is_indexed(function));
> 
> The problem with rejecting userspace CPUID on mismatch is that it could break
> userspace :-/  Of course, this entire patch would also break userspace to some
> extent, e.g. if userspace is relying on an exact match on index==0.  The only
> difference being the guest lookups with an exact index would still work.
> 
> I think the restriction we could put in place would be that userspace can make
> a leaf more relaxed, e.g. to play nice if userspace forgets to set the SIGNFICANT
> flag, but rejects attempts to make guest CPUID more restrictive, i.e. disallow
> setting the SIGNFICANT flag on leafs that KVM doesn't enumerate as significant.
> 
> > > +               WARN_ON_ONCE(index == KVM_CPUID_INDEX_NOT_SIGNIFICANT);

Actually, better idea.  Let userspace do whatever, and have direct KVM lookups
for functions that architecturally don't have a significant index use the first
entry even if userspace set the SIGNIFICANT flag.  That will mostly maintain
backwards compatibility, the only thing that would break is if userspace set the
SIGNIFICANT flag _and_ provided a non-zero index _and_ relied on KVM to not match
the entry.

We could still enforce matching in the future, but it wouldn't be a prerequisite
for this cleanup.

		/*
		 * Similarly, use the first matching entry if KVM is doing a
		 * lookup (as opposed to emulating CPUID) for a function that's
		 * architecturally defined as not having a significant index.
		 */
		if (index == KVM_CPUID_INDEX_NOT_SIGNIFICANT) {
			/*
			 * Direct lookups from KVM should not diverge from what
			 * KVM defines internally (the architectural behavior).
			 */
			WARN_ON_ONCE(cpuid_function_is_indexed(function));
			return e;
		}