[PATCH v2 3/4] 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>
---
v2:
  * New addition
---
 xen/arch/x86/cpu/microcode/core.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu/microcode/core.c 
b/xen/arch/x86/cpu/microcode/core.c
index 892bcec901..4f60d96d98 100644
--- a/xen/arch/x86/cpu/microcode/core.c
+++ b/xen/arch/x86/cpu/microcode/core.c
@@ -874,6 +874,21 @@ int __init early_microcode_init(unsigned long *module_map,
         break;
     }
 
+    if ( ucode_ops.collect_cpu_info )
+        ucode_ops.collect_cpu_info();
+
+    /*
+     * This is a special case for virtualized Xen. 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 )
+    {
+        this_cpu(cpu_sig) = (struct cpu_signature){ 0 };
+        ucode_ops = (struct microcode_ops){ 0 };
+    }
+
     if ( !ucode_ops.apply_microcode )
     {
         printk(XENLOG_WARNING "Microcode loading not available\n");
@@ -882,8 +897,6 @@ int __init early_microcode_init(unsigned long *module_map,
 
     microcode_grab_module(module_map, mbi);
 
-    ucode_ops.collect_cpu_info();
-
     if ( ucode_mod.mod_end || ucode_blob.size )
         rc = early_microcode_update_cpu();
 
-- 
2.34.1