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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.03.2025 00:33, Stefano Stabellini wrote:
> On Mon, 24 Mar 2025, Jan Beulich wrote:
>> On 22.03.2025 00:09, Stefano Stabellini wrote:
>>> @@ -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).
> 
> 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".

Jan