[PATCH 1/4] x86: Reject CPU policies with vendors other than the host's

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

While in principle it's possible to have a vendor virtualising another,
this is fairly tricky in practice and comes with the world's supply of
security issues.

Reject any CPU policy with vendors not matching the host's.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
---
 CHANGELOG.md         | 4 ++++
 xen/lib/x86/policy.c | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 18f3d10f20..eae2f961c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,10 @@ The format is based on [Keep a 
Changelog](https://keepachangelog.com/en/1.0.0/)
    - Xenoprofile support.  Oprofile themselves removed support for Xen in 2014
      prior to the version 1.0 release, and there has been no development since
      before then in Xen.
+   - Cross-vendor support.  Refuse to start domains whose CPU vendor differs
+     from the host so that security mitigations stay consistent. Cross-vendor
+     setups have been unreliable and not practical since 2017 with the advent 
of
+     speculation security.
 
  - Removed xenpm tool on non-x86 platforms as it doesn't actually provide
    anything useful outside of x86.
diff --git a/xen/lib/x86/policy.c b/xen/lib/x86/policy.c
index f033d22785..4c0c5386ea 100644
--- a/xen/lib/x86/policy.c
+++ b/xen/lib/x86/policy.c
@@ -15,7 +15,8 @@ int x86_cpu_policies_are_compatible(const struct cpu_policy 
*host,
 #define FAIL_MSR(m) \
     do { e.msr = (m); goto out; } while ( 0 )
 
-    if ( guest->basic.max_leaf > host->basic.max_leaf )
+    if ( (guest->basic.max_leaf >  host->basic.max_leaf) ||
+         (guest->x86_vendor     != host->x86_vendor) )
         FAIL_CPUID(0, NA);
 
     if ( guest->feat.max_subleaf > host->feat.max_subleaf )
-- 
2.43.0