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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.03.2025 00:09, Stefano Stabellini wrote:
> 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];

Same style nit as before.

> +        if ( copy_bytes &&

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 &&
> +             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).

> +    }
> +
> +    clamp_last_byte(&last, nbits);
> +    if ( copy_to_guest_offset(xenctl_bitmap->bitmap, copy_bytes - 1, &last, 
> 1) )
> +        err = -EFAULT;

Careful here in particular when copy_bytes == 0.

Jan