[PATCH] x86/cpuid: do not expand max leaves on restore

[PATCH] x86/cpuid: do not expand max leaves on restore

[Roger Pau Monne]

When restoring limit the maximum leaves to the ones supported by Xen
4.13 in order to not expand the maximum leaf a guests sees. Note this
is unlikely to cause real issues.

Guests restored from Xen versions 4.13 or greater will contain CPUID
data on the stream that will override the values set by
xc_cpuid_apply_policy.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
 tools/libs/guest/xg_cpuid_x86.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 5ea69ad3d51..9296fdc34bd 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -510,6 +510,11 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
         {
             p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
         }
+
+        /* Clamp maximum leaves to the supported ones on 4.13. */
+        p->basic.max_leaf = min(p->basic.max_leaf, 0xdu);
+        p->feat.max_subleaf = min(p->feat.max_subleaf, 1u);
+        p->extd.max_leaf = min(p->extd.max_leaf, 0x1cu);
     }
 
     if ( featureset )
-- 
2.30.1

Re: [PATCH] x86/cpuid: do not expand max leaves on restore

[Jan Beulich]

On 23.04.2021 12:54, Roger Pau Monne wrote:
> When restoring limit the maximum leaves to the ones supported by Xen
> 4.13 in order to not expand the maximum leaf a guests sees. Note this
> is unlikely to cause real issues.

Why 4.13 (and not 4.12) here when ...

> Guests restored from Xen versions 4.13 or greater will contain CPUID
> data on the stream that will override the values set by
> xc_cpuid_apply_policy.

... 4.13 already communicates the values?

> --- a/tools/libs/guest/xg_cpuid_x86.c
> +++ b/tools/libs/guest/xg_cpuid_x86.c
> @@ -510,6 +510,11 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
>          {
>              p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
>          }
> +
> +        /* Clamp maximum leaves to the supported ones on 4.13. */

Same aspect here then. (While not being a native speaker, it
would still seem to me that flipping "supported" and "ones"
would read slightly more clearly.)

> +        p->basic.max_leaf = min(p->basic.max_leaf, 0xdu);
> +        p->feat.max_subleaf = min(p->feat.max_subleaf, 1u);
> +        p->extd.max_leaf = min(p->extd.max_leaf, 0x1cu);
>      }

With this I think the comment ahead of the enclosing if() wants
either amending or moving immediately inside the if()'s body.

Jan

Re: [PATCH] x86/cpuid: do not expand max leaves on restore

[Roger Pau Monné]

On Fri, Apr 23, 2021 at 02:41:32PM +0200, Jan Beulich wrote:
> On 23.04.2021 12:54, Roger Pau Monne wrote:
> > When restoring limit the maximum leaves to the ones supported by Xen
> > 4.13 in order to not expand the maximum leaf a guests sees. Note this
> > is unlikely to cause real issues.
> 
> Why 4.13 (and not 4.12) here when ...
> 
> > Guests restored from Xen versions 4.13 or greater will contain CPUID
> > data on the stream that will override the values set by
> > xc_cpuid_apply_policy.
> 
> ... 4.13 already communicates the values?

Urg, yes, my bad. This should have been limited to the max leaves
supported by 4.12, not 4.13.

Let me send a new version also incorporating the feedback from below.

Thanks, Roger.