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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 28 Mar 2025, Jan Beulich wrote:
> >>> +             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;
> >>
> >> The two identical instances would imo better stay common, even if this may
> >> require another function-scope variable (to invoke xfree() on after the
> >> copy-out).
> > 
> > That's not possible because bytemap is defined differently in the two
> > cases so it has to be defined within the if block.
> 
> Hence why I said "even if this may require another function-scope variable".

Sorry Jan, I don't understand in practice how to make your suggestion
work.

Do you mean to do this?

#ifdef __BIG_ENDIAN
        uint8_t *bytemap;
#else
        const uint8_t *bytemap;
#endif

If so, I get this build error:

common/bitmap.c: In function ‘bitmap_to_xenctl_bitmap’:
common/bitmap.c:402:29: error: passing argument 1 of ‘bitmap_long_to_byte’ 
discards ‘const’ qualifier from pointer target type 
[-Werror=discarded-qualifiers]
  402 |         bitmap_long_to_byte(bytemap, bitmap, nbits);
      |                             ^~~~~~~

I am trying to guess what you have in mind, but it might be more
effective if you could write it down. For reference, I am including the
patch I was trying to get to work.



diff --git a/xen/common/bitmap.c b/xen/common/bitmap.c
index 3da63a32a6..b213e29ca5 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 +382,43 @@ 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;
+#ifdef __BIG_ENDIAN
+       uint8_t *bytemap;
+#else
+       const uint8_t *bytemap;
+#endif
 
     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) )
+    {
+        bytemap = xmalloc_array(uint8_t, xen_bytes);
+
+        if ( !bytemap )
+            return -ENOMEM;
 
-    if ( copy_bytes &&
-         copy_to_guest(xenctl_bitmap->bitmap, bytemap, copy_bytes) )
+        bitmap_long_to_byte(bytemap, bitmap, nbits);
+        last = bytemap[nbits / 8];
+    }
+    else
+    {
+        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;
 
-    xfree(bytemap);
+    if ( IS_ENABLED(__BIG_ENDIAN) )
+        xfree(bytemap);
+
+    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) )