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

[patchbot@xxxxxxx]

commit 111c8c33a8a18588f3da3c5dbb7f5c63ddb98ce5
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Thu Apr 29 16:04:11 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 29 16:04:11 2021 +0200

    x86/cpuid: do not expand max leaves on restore
    
    When restoring limit the maximum leaves to the ones supported by Xen
    4.12 in order to not expand the maximum leaves 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@xxxxxxxxxx>
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 tools/libs/guest/xg_cpuid_x86.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 5ea69ad3d5..bf9a3750b5 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -498,18 +498,23 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
domid, bool restore,
         goto out;
     }
 
-    /*
-     * Account for feature which have been disabled by default since Xen 4.13,
-     * so migrated-in VM's don't risk seeing features disappearing.
-     */
     if ( restore )
     {
+        /*
+         * Account for feature which have been disabled by default since Xen 
4.13,
+         * so migrated-in VM's don't risk seeing features disappearing.
+         */
         p->basic.rdrand = test_bit(X86_FEATURE_RDRAND, host_featureset);
 
         if ( di.hvm )
         {
             p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
         }
+
+        /* Clamp maximum leaves to the ones supported on 4.12. */
+        p->basic.max_leaf = min(p->basic.max_leaf, 0xdu);
+        p->feat.max_subleaf = 0;
+        p->extd.max_leaf = min(p->extd.max_leaf, 0x1cu);
     }
 
     if ( featureset )
--
generated by git-patchbot for /home/xen/git/xen.git#master