diff mbox series

x86/cpuid: do not expand max leaves on restore

Message ID 20210423105408.7265-1-roger.pau@citrix.com (mailing list archive)
State Superseded
Headers show
Series x86/cpuid: do not expand max leaves on restore | expand

Commit Message

Roger Pau Monné April 23, 2021, 10:54 a.m. UTC
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(+)

Comments

Jan Beulich April 23, 2021, 12:41 p.m. UTC | #1
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
Roger Pau Monné April 23, 2021, 12:47 p.m. UTC | #2
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.
diff mbox series

Patch

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 )