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[Xen-devel] [PATCH v2 2/4] x86/microcode: avoid unnecessary xmalloc/memcpy of ucode data

When using `ucode=scan` and if a matching module is found, the microcode
payload is maintained in an xmalloc()'d region. This is unnecessary since
the bootmap would just do. Remove the xmalloc and xfree on the microcode
module scan path.

This commit also does away with the restriction on the microcode module
size limit. The concern that a large microcode module would consume too
much memory preventing guests launch is misplaced since this is all the
init path. While having such safeguards is valuable, this should apply
across the board for all early/late microcode loading. Having it just on
the `scan` path is confusing.

Looking forward, we are a bit closer (i.e., one xmalloc down) to pulling
the early microcode loading of the BSP a bit earlier in the early boot
process. This commit is the low hanging fruit. There is still a sizable
amount of work to get there as there are still a handful of xmalloc in

First, there are xmallocs on the path of finding a matching microcode
update. Similar to the commit at hand, searching through the microcode
blob can be done on the already present buffer with no need to xmalloc
any further. Even better, do the filtering in microcode.c before
requesting the microcode update on all CPUs. The latter requires careful
restructuring and exposing the arch-specific logic for iterating over
patches and declaring a match.

Second, there are xmallocs for the microcode cache. Here, we would need
to ensure that the cache corresponding to the BSP gets xmalloc()'d and
populated after the fact.

Signed-off-by: Eslam Elnikety <elnikety@xxxxxxxxxx>
 xen/arch/x86/microcode.c | 32 ++++----------------------------
 1 file changed, 4 insertions(+), 28 deletions(-)

diff --git a/xen/arch/x86/microcode.c b/xen/arch/x86/microcode.c
index 8b4d87782c..c878fc71ff 100644
--- a/xen/arch/x86/microcode.c
+++ b/xen/arch/x86/microcode.c
@@ -138,11 +138,6 @@ static int __init parse_ucode_param(const char *s)
 custom_param("ucode", parse_ucode_param);
- * 8MB ought to be enough.
- */
-#define MAX_EARLY_CPIO_MICROCODE (8 << 20)
 void __init microcode_scan_module(
     unsigned long *module_map,
     const multiboot_info_t *mbi)
@@ -187,31 +182,12 @@ void __init microcode_scan_module(
         cd = find_cpio_data(p, _blob_start, _blob_size, &offset /* ignore */);
         if ( cd.data )
-                /*
-                 * This is an arbitrary check - it would be sad if the blob
-                 * consumed most of the memory and did not allow guests
-                 * to launch.
-                 */
-                if ( cd.size > MAX_EARLY_CPIO_MICROCODE )
-                {
-                    printk("Multiboot %d microcode payload too big! (%ld, we 
can do %d)\n",
-                           i, cd.size, MAX_EARLY_CPIO_MICROCODE);
-                    goto err;
-                }
-                ucode_blob.size = cd.size;
-                ucode_blob.data = xmalloc_bytes(cd.size);
-                if ( !ucode_blob.data )
-                    cd.data = NULL;
-                else
-                    memcpy(ucode_blob.data, cd.data, cd.size);
+            ucode_blob.size = cd.size;
+            ucode_blob.data = cd.data;
+            break;
-        if ( cd.data )
-            break;
-    return;
-    bootstrap_map(NULL);
 void __init microcode_grab_module(
     unsigned long *module_map,
@@ -725,7 +701,7 @@ static int __init microcode_init(void)
     if ( ucode_blob.size )
-        xfree(ucode_blob.data);
+        bootstrap_map(NULL);
         ucode_blob.size = 0;
         ucode_blob.data = NULL;

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