x86/ucode: Further fixes to identify "ucode already up to date"

When the revision in hardware is newer than anything Xen has to hand,
'microcode_cache' isn't set up.  Then, `xen-ucode` initiates the update
because it doesn't know whether the revisions across the system are symmetric
or not.  This involves the patch getting all the way into the
apply_microcode() hooks before being found to be too old.

This is all a giant mess and needs an overhaul, but in the short term simply
adjust the apply_microcode() to return -EEXIST.

Also, unconditionally print the preexisting microcode revision on boot.  It's
relevant information which is otherwise unavailable if Xen doesn't find new
microcode to use.

Fixes: 648db37a15 ("x86/ucode: Distinguish "ucode already up to date"")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 977d98e67c
master date: 2024-05-16 13:59:11 +0100

xen/arch/x86/cpu/microcode/amd.c

@@ -222,12 +222,15 @@ static int cf_check apply_microcode(const struct microcode_patch *patch)
     uint32_t rev, old_rev = sig->rev;
     enum microcode_match_result result = microcode_fits(patch);
 
+    if ( result == MIS_UCODE )
+        return -EINVAL;
+
     /*
      * Allow application of the same revision to pick up SMT-specific changes
      * even if the revision of the other SMT thread is already up-to-date.
      */
-    if ( result != NEW_UCODE && result != SAME_UCODE )
-        return -EINVAL;
+    if ( result == OLD_UCODE )
+        return -EEXIST;
 
     if ( check_final_patch_levels(sig) )
     {

xen/arch/x86/cpu/microcode/core.c

@@ -819,6 +819,8 @@ int __init early_microcode_init(void)
 
     alternative_vcall(ucode_ops.collect_cpu_info);
 
+    printk(XENLOG_INFO "BSP microcode revision: 0x%08x\n", this_cpu(cpu_sig).rev);
+
     if ( ucode_mod.mod_end || ucode_blob.size )
         rc = early_microcode_update_cpu();
 

xen/arch/x86/cpu/microcode/intel.c

@@ -294,10 +294,13 @@ static int cf_check apply_microcode(const struct microcode_patch *patch)
 
     result = microcode_update_match(patch);
 
-    if ( result != NEW_UCODE &&
-         !(opt_ucode_allow_same && result == SAME_UCODE) )
+    if ( result == MIS_UCODE )
         return -EINVAL;
 
+    if ( result == OLD_UCODE ||
+         (result == SAME_UCODE && !opt_ucode_allow_same) )
+        return -EEXIST;
+
     wbinvd();
 
     wrmsrl(MSR_IA32_UCODE_WRITE, (unsigned long)patch->data);