[xen staging] x86/ucode/intel: Fix handling of microcode revision

[patchbot@xxxxxxx]

commit aa1b136ff1798a2c1a81270ccd20e7252e30ff1f
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Oct 26 15:27:35 2020 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Jan 27 20:29:43 2021 +0000

    x86/ucode/intel: Fix handling of microcode revision
    
    For Intel microcode blobs, the revision field is signed (as documented in 
the
    SDM) and negative revisions are used for pre-production/test microcode (not
    documented publicly anywhere I can spot).
    
    Adjust the revision checking to match the algorithm presented here:
    
      
https://software.intel.com/security-software-guidance/best-practices/microcode-update-guidance
    
    This treats pre-production microcode as always applicable, but also 
production
    microcode having higher precedent than pre-production.  It is expected that
    anyone using pre-production microcode knows what they are doing.
    
    This is necessary to load production microcode on an SDP with pre-production
    microcode embedded in firmware.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/cpu/microcode/intel.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu/microcode/intel.c 
b/xen/arch/x86/cpu/microcode/intel.c
index e1ccb5d232..5fa2821cdb 100644
--- a/xen/arch/x86/cpu/microcode/intel.c
+++ b/xen/arch/x86/cpu/microcode/intel.c
@@ -33,7 +33,7 @@
 
 struct microcode_patch {
     uint32_t hdrver;
-    uint32_t rev;
+    int32_t rev;
     uint16_t year;
     uint8_t  day;
     uint8_t  month;
@@ -222,12 +222,23 @@ static int microcode_sanity_check(const struct 
microcode_patch *patch)
     return 0;
 }
 
+/*
+ * Production microcode has a positive revision.  Pre-production microcode has
+ * a negative revision.
+ */
 static enum microcode_match_result compare_revisions(
-    uint32_t old_rev, uint32_t new_rev)
+    int32_t old_rev, int32_t new_rev)
 {
     if ( new_rev > old_rev )
         return NEW_UCODE;
 
+    /*
+     * Treat pre-production as always applicable - anyone using pre-production
+     * microcode knows what they are doing, and can keep any resulting pieces.
+     */
+    if ( new_rev < 0 )
+        return NEW_UCODE;
+
     return OLD_UCODE;
 }
 
--
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