x86: Reject CPU policies with vendors other than the host's

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@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com> # CHANGELOG.md

CHANGELOG.md

@@ -31,6 +31,12 @@ 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; guests can now only be configured as the same
+     vendor as the host CPU.  When added back in 2009, with enough trickery
+     Intel and AMD CPUs could be made to be compatible enough to live migrate
+     a guest, but the vendors have been diverging since then in ways that Xen
+     cannot compensate for, and the advent of speculative security issues has
+     put to rest any possibility of this being a viable option.
 
  - Removed xenpm tool on non-x86 platforms as it doesn't actually provide
    anything useful outside of x86.

tools/tests/cpu-policy/test-cpu-policy.c

@@ -586,6 +586,19 @@ static void test_is_compatible_success(void)
                 .platform_info.cpuid_faulting = true,
             },
         },
+        {
+            .name = "Host CPU vendor == Guest CPU vendor (both unknown)",
+            .host = {
+                .basic.vendor_ebx = X86_VENDOR_AMD_EBX + 1,
+                .basic.vendor_ecx = X86_VENDOR_AMD_ECX,
+                .basic.vendor_edx = X86_VENDOR_AMD_EDX,
+            },
+            .guest = {
+                .basic.vendor_ebx = X86_VENDOR_AMD_EBX + 1,
+                .basic.vendor_ecx = X86_VENDOR_AMD_ECX,
+                .basic.vendor_edx = X86_VENDOR_AMD_EDX,
+            },
+        },
     };
     struct cpu_policy_errors no_errors = INIT_CPU_POLICY_ERRORS;
 
@@ -629,6 +642,20 @@ static void test_is_compatible_failure(void)
             },
             .e = { -1, -1, 0xce },
         },
+        {
+            .name = "Host CPU vendor != Guest CPU vendor (both unknown)",
+            .host = {
+                .basic.vendor_ebx = X86_VENDOR_AMD_EBX + 1,
+                .basic.vendor_ecx = X86_VENDOR_AMD_ECX,
+                .basic.vendor_edx = X86_VENDOR_AMD_EDX,
+            },
+            .guest = {
+                .basic.vendor_ebx = X86_VENDOR_AMD_EBX + 2,
+                .basic.vendor_ecx = X86_VENDOR_AMD_ECX,
+                .basic.vendor_edx = X86_VENDOR_AMD_EDX,
+            },
+            .e = { 0, -1, -1 },
+        },
     };
 
     printf("Testing policy compatibility failure:\n");

xen/arch/x86/lib/cpu-policy/policy.c

@@ -15,7 +15,10 @@ 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.vendor_ebx != host->basic.vendor_ebx) ||
+         (guest->basic.vendor_ecx != host->basic.vendor_ecx) ||
+         (guest->basic.vendor_edx != host->basic.vendor_edx) ||
+         (guest->basic.max_leaf   >  host->basic.max_leaf) )
         FAIL_CPUID(0, NA);
 
     if ( guest->feat.max_subleaf > host->feat.max_subleaf )