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>
---
v4:
  * Adjusted CHANGELOG
---
 CHANGELOG.md                             |  5 +++++
 tools/tests/cpu-policy/test-cpu-policy.c | 27 ++++++++++++++++++++++++
 xen/arch/x86/lib/cpu-policy/policy.c     |  5 ++++-
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c191e504aba..90ba5da69e4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,11 @@ 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.
+   - Domains can no longer run on a system with CPUs of a vendor different from
+     the one they were initially launched on. This affects live migrations and
+     save/restore workflows across mixed-vendor hosts. Cross-vendor emulation
+     has always been unreliable, but since 2017 with the advent of speculation
+     security it became unsustainably so.
 
  - 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 301df2c0028..88a9a26e8f1 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 f033d22785b..f991b1f3a96 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 )
-- 
2.43.0

On 11.03.2026 15:27, Alejandro Vallejo wrote:
> 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>