[PATCH] tools/guest: Fix comment regarding CPUID compatibility

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>

Roger, this also has a knock-on effect on your CPUID series.  The 4.12 had
been nagging me for a while before I figured out why.
---
 tools/libs/guest/xg_cpuid_x86.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index b9e827ce7eb0..57f81eb0a082 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -497,9 +497,19 @@ 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);
@@ -509,7 +519,6 @@ 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);
-- 
2.11.0