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
