Re: [PATCH v3] xen: simplify bitmap_to_xenctl_bitmap for little endian

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.03.2025 00:34, Stefano Stabellini wrote:
> --- 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;
>  }

Technically there's nothing wrong with dropping the if(), I think. Even more
so then than now, ...

> @@ -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);
>  }

... with the two prior call sites now shrunk to ...

> @@ -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);

... just one, I wonder if that being a separate function is actually still
necessary.

As indicated before, I think it would be nice if the two identical copy-out
operations could also be folded.

Jan