[xen master] x86: Reject CPU policies with vendors other than the host's

[patchbot@xxxxxxx]

commit c520286a5380b0133213b53480fb2a4c2d631fc4
Author:     Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
AuthorDate: Thu Mar 12 12:21:10 2026 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 9 08:22:36 2026 +0200

    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@xxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx> # CHANGELOG.md
---
 CHANGELOG.md                             |  6 ++++++
 tools/tests/cpu-policy/test-cpu-policy.c | 27 +++++++++++++++++++++++++++
 xen/arch/x86/lib/cpu-policy/policy.c     |  5 ++++-
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 166df43c63..47b6021ce7 100644
--- a/CHANGELOG.md
+++ b/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.
diff --git a/tools/tests/cpu-policy/test-cpu-policy.c 
b/tools/tests/cpu-policy/test-cpu-policy.c
index 301df2c002..88a9a26e8f 100644
--- a/tools/tests/cpu-policy/test-cpu-policy.c
+++ b/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");
diff --git a/xen/arch/x86/lib/cpu-policy/policy.c 
b/xen/arch/x86/lib/cpu-policy/policy.c
index f033d22785..f991b1f3a9 100644
--- a/xen/arch/x86/lib/cpu-policy/policy.c
+++ b/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 )
--
generated by git-patchbot for /home/xen/git/xen.git#master