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

[Julien Grall <julien@xxxxxxx>]

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 <stefano.stabellini@xxxxxxx>
> ---
> 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,

--
Julien Grall