x86/ucode: Support discrete modules being CPIO archives

Multiple downstream distros have tried passing discrete CPIO archives and
tripped over this not working.  It turns out to be easy to support, so do so.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Teddy Astie <teddy.astie@vates.tech>
[jb: doc change dropped as inapplicable to the older form]
master commit: 878a9cfa45
master date: 2026-03-03 15:15:53 +0000

CHANGELOG.md

@@ -4,6 +4,12 @@ Notable changes to Xen will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
+## [4.21.1](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.21.1)
+
+### Added
+ - On x86:
+   - Support for CPIO microcode in discrete multiboot modules.
+
 ## [4.21.0](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.21.0) - 2025-11-19
 
 ### Changed

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

@@ -749,6 +749,7 @@ static int __init early_microcode_load(struct boot_info *bi)
     void *data = NULL;
     size_t size;
     struct microcode_patch *patch;
+    struct cpio_data cd;
     int idx = opt_mod_idx;
     int rc;
 
@@ -765,7 +766,6 @@ static int __init early_microcode_load(struct boot_info *bi)
         for ( idx = 0; idx < bi->nr_modules; ++idx )
         {
             const struct boot_module *bm = &bi->mods[idx];
-            struct cpio_data cd;
 
             /* Search anything unclaimed or likely to be a CPIO archive. */
             if ( bm->kind != BOOTMOD_UNKNOWN && bm->kind != BOOTMOD_RAMDISK )
@@ -833,6 +833,24 @@ static int __init early_microcode_load(struct boot_info *bi)
                    idx, size);
             return -ENODEV;
         }
+
+        /*
+         * If this blob appears to be a CPIO archive, try interpreting it as
+         * one.  Otherwise treat it as a raw vendor blob.
+         */
+        cd = find_cpio_data(ucode_ops.cpio_path, data, size);
+        if ( cd.data )
+        {
+            data = cd.data;
+            size = cd.size;
+
+            /*
+             * (Ab)use opt_scan to inform microcode_init_cache() that
+             * early_mod_idx refers to a CPIO archive.
+             */
+            opt_scan = true;
+        }
+
         goto found;
     }