[PATCH v7 1/3] x86/microcode: Ignore microcode loading interface for revision = -1

[Alejandro Vallejo <alejandro.vallejo@cloud.com>]

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@cloud.com>
---
v7:
  * Removed R-by tag. It wasn't meant to be there!
  * Fix wrong substring inserted in printk statement
  * Returned the printk severity to WARNING

I still think both this new print and the previous one in the function
should have severity INFO instead, but I'm not bothered either way and they
should both have the same severity because they highlight equivalent events.
---
 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 bec8b55db2..205bb52201 100644
--- a/xen/arch/x86/cpu/microcode/core.c
+++ b/xen/arch/x86/cpu/microcode/core.c
@@ -867,10 +867,23 @@ 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 due to: %s",
+                              "rev = ~0");
+        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