tools/guest: Fix comment regarding CPUID compatibility

It was Xen 4.14 where CPUID data was added to the migration stream, and 4.13
that we need to worry about with regards to compatibility.  Xen 4.12 isn't
relevant.

Expand and correct the commentary.

Fixes: 111c8c33a8a1 ("x86/cpuid: do not expand max leaves on restore")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 820cc393434097f3b7976acdccbf1d96071d6d23)

diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 198892ebdf45d7aeb37720995ceabff19bf7b027..6ea14c4c56eabf214efb1e43c142b74a41578429 100644 (file)
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -498,9 +498,19 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
     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.
+         * Xen 4.14 introduced support to move the guest's CPUID data in the
+         * migration stream.  Previously, the destination side would invent a
+         * policy out of thin air in the hopes that it was ok.
+         *
+         * This restore path is used for incoming VMs with no CPUID data
+         * i.e. originated on Xen 4.13 or earlier.  We must invent a policy
+         * compatible with what Xen 4.13 would have done on the same hardware.
+         *
+         * Specifically:
+         * - Clamp max leaves.
+         * - Re-enable features which have become (possibly) off by default.
          */
+
         p->basic.rdrand = test_bit(X86_FEATURE_RDRAND, host_featureset);
         p->feat.hle = test_bit(X86_FEATURE_HLE, host_featureset);
         p->feat.rtm = test_bit(X86_FEATURE_RTM, host_featureset);
@@ -510,7 +520,6 @@ 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 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, 0x8000001c);