Re: [PATCH] xen: simplify bitmap_to_xenctl_bitmap for little endian

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 19 Mar 2025, Jan Beulich wrote:
> What about xenctl_bitmap_to_bitmap()?
 
Let me see first if I managed to handle bitmap_to_xenctl_bitmap well.

---

[PATCH v2] xen: simplify bitmap_to_xenctl_bitmap for little endian

The little endian implementation of bitmap_to_xenctl_bitmap leads to
unnecessary xmallocs and xfrees. Given that Xen only supports little
endian architectures, it is worth optimizing.

This patch removes the need for the xmalloc on little endian
architectures.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
---
Changes in v2:
- don't remove const
- handle clamp_last_byte for little endian

diff --git a/xen/common/bitmap.c b/xen/common/bitmap.c
index 3da63a32a6..e9876ee5a6 100644
--- a/xen/common/bitmap.c
+++ b/xen/common/bitmap.c
@@ -384,21 +384,33 @@ int bitmap_to_xenctl_bitmap(struct xenctl_bitmap 
*xenctl_bitmap,
     uint8_t zero = 0;
     int err = 0;
     unsigned int xen_bytes = DIV_ROUND_UP(nbits, BITS_PER_BYTE);
-    uint8_t *bytemap = xmalloc_array(uint8_t, xen_bytes);
-
-    if ( !bytemap )
-        return -ENOMEM;
+    bool alloc = (bitmap[nbits/8] & ((1U << (nbits % 8)) - 1)) ||
+                 IS_ENABLED(__BIG_ENDIAN);
 
     guest_bytes = DIV_ROUND_UP(xenctl_bitmap->nr_bits, BITS_PER_BYTE);
     copy_bytes  = min(guest_bytes, xen_bytes);
 
-    bitmap_long_to_byte(bytemap, bitmap, nbits);
+    if ( alloc )
+    {
+        uint8_t *bytemap = xmalloc_array(uint8_t, xen_bytes);
+        if ( !bytemap )
+            return -ENOMEM;
 
-    if ( copy_bytes &&
-         copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes) )
-        err = -EFAULT;
+        bitmap_long_to_byte(bytemap, bitmap, nbits);
 
-    xfree(bytemap);
+        if ( copy_bytes &&
+             copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes) )
+            err = -EFAULT;
+
+        xfree(bytemap);
+    }
+    else
+    {
+        const uint8_t *bytemap = (const uint8_t *)bitmap;
+        if ( copy_bytes &&
+             copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes) )
+            err = -EFAULT;
+    }
 
     for ( i = copy_bytes; !err && i < guest_bytes; i++ )
         if ( copy_to_guest_offset(xenctl_bitmap->bitmap, i, &zero, 1) )