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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 31 Mar 2025, Jan Beulich wrote:

> 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.
 
Please see: https://marc.info/?l=xen-devel&m=174319650100975

Other than that, I addressed all the other points