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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 20 Mar 2025, Jan Beulich wrote:
> On 20.03.2025 01:57, Stefano Stabellini wrote:
> > 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.
> 
> Well, the code looks correct to me, but the description now has gone
> stale. I also wonder whether with that extra restriction the optimization
> is then actually worth it. Just one further nit:

Hi Jan, you make a good point. I tried to come up with a better
approach. What do you think of this?


diff --git a/xen/common/bitmap.c b/xen/common/bitmap.c
index 3da63a32a6..2f448693c3 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 +382,40 @@ 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 &&
+             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 &&
+             copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes - 1) )
+            err = -EFAULT;
+    }
+
+    clamp_last_byte(&last, nbits);
+    if ( 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) )