Re: [PATCH for-4.16] x86/cpuid: prevent shrinking migrated policies max leaves

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 11, 2021 at 10:26:29AM +0100, Jan Beulich wrote:
> On 10.11.2021 19:17, Andrew Cooper wrote:
> > On 10/11/2021 17:40, Roger Pau Monne wrote:
> >> diff --git a/tools/libs/guest/xg_cpuid_x86.c 
> >> b/tools/libs/guest/xg_cpuid_x86.c
> >> index 198892ebdf..3ffd5f683b 100644
> >> --- a/tools/libs/guest/xg_cpuid_x86.c
> >> +++ b/tools/libs/guest/xg_cpuid_x86.c
> >> @@ -638,6 +638,13 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
> >> domid, bool restore,
> >>          }
> >>      }
> >>  
> >> +    /*
> >> +     * Do not try to shrink the policy if restoring, as that could cause
> >> +     * guest visible changes in the maximum leaf fields.
> >> +     */
> >> +    if ( !restore )
> >> +        x86_cpuid_policy_shrink_max_leaves(p);
> > 
> > Nothing in xc_cpuid_apply_policy() changes any of the max leaves, so
> > this is dead logic.
> 
> I guess you mean nothing there does anything which would result in
> shrinking of the max leaves by calling this function? Yet even if
> so, isn't the call here to take care of any earlier changes which
> might have resulted in fully blank tail leaves?

AFAICT the featureset (optionally) passed in as a parameter could
result in certain leaves being zeroed and thus allow for the max leaf
to shrink.

So while xc_cpuid_apply_policy doesn't change the max leaves fields,
it can potentially zero certain leaves allowing to shrink the reported
max leaf.

Thanks, Roger.