[v3] xen: simplify bitmap_to_xenctl_bitmap for little endian

Commit Message

Stefano Stabellini March 27, 2025, 11:34 p.m. UTC

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 <[e-mail]>
---
Changes in v3:
- code style
- copy_bytes > 1 checks
- copy_bytes > 0 check for copy_to_guest_offset
---
 xen/common/bitmap.c | 42 ++++++++++++++++++++++++++++++------------
 1 file changed, 30 insertions(+), 12 deletions(-)

Comments

Julien Grall March 30, 2025, 9:29 a.m. UTC | #1

Hi Stefano,

On 27/03/2025 23:34, Stefano Stabellini wrote:
> 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 <[e-mail]>
> ---
> Changes in v3:
> - code style
> - copy_bytes > 1 checks
> - copy_bytes > 0 check for copy_to_guest_offset
> ---
>   xen/common/bitmap.c | 42 ++++++++++++++++++++++++++++++------------
>   1 file changed, 30 insertions(+), 12 deletions(-)
> 
> diff --git a/xen/common/bitmap.c b/xen/common/bitmap.c
> index 3da63a32a6..d3f9347e62 100644
> --- a/xen/common/bitmap.c
> +++ b/xen/common/bitmap.c
> @@ -52,7 +52,7 @@ static void clamp_last_byte(uint8_t *bp, unsigned int nbits)
>   	unsigned int remainder = nbits % 8;
>   
>   	if (remainder)
> -		bp[nbits/8] &= (1U << remainder) - 1;
> +		*bp &= (1U << remainder) - 1;

This is changing the behavior of clamp_last_byte(). Yet, it doesn't seem 
the comment on top of is changed. Is this intended? Also, I would 
consider rename 'bp' so it is clearer this is meant to point to the last 
byte of the bitmap rather than the start.

Cheers,

Patch

diff --git a/xen/common/bitmap.c b/xen/common/bitmap.c
index 3da63a32a6..d3f9347e62 100644
--- a/xen/common/bitmap.c
+++ b/xen/common/bitmap.c
@@ -52,7 +52,7 @@  static void clamp_last_byte(uint8_t *bp, unsigned int nbits)
 	unsigned int remainder = nbits % 8;
 
 	if (remainder)
-		bp[nbits/8] &= (1U << remainder) - 1;
+		*bp &= (1U << remainder) - 1;
 }
 
 int __bitmap_empty(const unsigned long *bitmap, unsigned int bits)
@@ -338,7 +338,6 @@  static void bitmap_long_to_byte(uint8_t *bp, const unsigned long *lp,
 			nbits -= 8;
 		}
 	}
-	clamp_last_byte(bp, nbits);
 }
 
 static void bitmap_byte_to_long(unsigned long *lp, const uint8_t *bp,
@@ -363,7 +362,6 @@  static void bitmap_long_to_byte(uint8_t *bp, const unsigned long *lp,
 				unsigned int nbits)
 {
 	memcpy(bp, lp, DIV_ROUND_UP(nbits, BITS_PER_BYTE));
-	clamp_last_byte(bp, nbits);
 }
 
 static void bitmap_byte_to_long(unsigned long *lp, const uint8_t *bp,
@@ -384,21 +382,41 @@  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;
+    uint8_t last;
 
     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 ( IS_ENABLED(__BIG_ENDIAN) )
+    {
+        uint8_t *bytemap = xmalloc_array(uint8_t, xen_bytes);
 
-    if ( copy_bytes &&
-         copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes) )
-        err = -EFAULT;
+        if ( !bytemap )
+            return -ENOMEM;
 
-    xfree(bytemap);
+        bitmap_long_to_byte(bytemap, bitmap, nbits);
+        last = bytemap[nbits / 8];
+
+        if ( copy_bytes > 1 &&
+             copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes - 1) )
+            err = -EFAULT;
+
+        xfree(bytemap);
+    }
+    else
+    {
+        const uint8_t *bytemap = (const uint8_t *)bitmap;
+        last = bytemap[nbits / 8];
+
+        if ( copy_bytes > 1 &&
+             copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes - 1) )
+            err = -EFAULT;
+    }
+
+    clamp_last_byte(&last, nbits);
+    if ( copy_bytes > 0 &&
+         copy_to_guest_offset(xenctl_bitmap->bitmap, copy_bytes - 1, &last, 1) )
+        err = -EFAULT;
 
     for ( i = copy_bytes; !err && i < guest_bytes; i++ )
         if ( copy_to_guest_offset(xenctl_bitmap->bitmap, i, &zero, 1) )