[PATCH v5 2/4] x86/microcode: Ignore microcode loading interface for revision = -1

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

Some hypervisors report ~0 as the microcode revision to mean "don't issue
microcode updates". Ignore the microcode loading interface in that case.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v5:
  * Style fix. Brace position.
---
 xen/arch/x86/cpu/microcode/core.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu/microcode/core.c 
b/xen/arch/x86/cpu/microcode/core.c
index bec8b55db2..b620e3bfa6 100644
--- a/xen/arch/x86/cpu/microcode/core.c
+++ b/xen/arch/x86/cpu/microcode/core.c
@@ -867,10 +867,22 @@ int __init early_microcode_init(unsigned long *module_map,
         return -ENODEV;
     }
 
-    microcode_grab_module(module_map, mbi);
-
     ucode_ops.collect_cpu_info();
 
+    /*
+     * Some hypervisors deliberately report a microcode revision of -1 to
+     * mean that they will not accept microcode updates. We take the hint
+     * and ignore the microcode interface in that case.
+     */
+    if ( this_cpu(cpu_sig).rev == ~0 )
+    {
+        printk(XENLOG_WARNING "Microcode loading disabled\n");
+        ucode_ops.apply_microcode = NULL;
+        return -ENODEV;
+    }
+
+    microcode_grab_module(module_map, mbi);
+
     if ( ucode_mod.mod_end || ucode_blob.size )
         rc = early_microcode_update_cpu();
 
-- 
2.34.1