[Libguestfs] [nbdkit PATCH] swab: Implement .block_size callback

[Eric Blake]

On Tue, Feb 22, 2022 at 12:49:13PM +0100, Laszlo Ersek wrote:
> On 02/21/22 23:00, Eric Blake wrote:
> > We were previously enforcing minimum block size with EINVAL for
> > too-small requests.  Advertise this to the client.
> > ---
> >  filters/swab/nbdkit-swab-filter.pod |  6 ++++++
> >  filters/swab/swab.c                 | 24 +++++++++++++++++++++++-
> >  2 files changed, 29 insertions(+), 1 deletion(-)
> > 
> > diff --git a/filters/swab/nbdkit-swab-filter.pod b/filters/swab/nbdkit-swab-filter.pod
> > index f8500150..030a0852 100644
> > --- a/filters/swab/nbdkit-swab-filter.pod
> > +++ b/filters/swab/nbdkit-swab-filter.pod
> > @@ -34,6 +34,11 @@ the last few bytes, combine this filter with
> >  L<nbdkit-truncate-filter(1)>; fortunately, sector-based disk images
> >  are already suitably sized.
> > 
> > +Note that this filter fails operations that are not aligned to the
> > +swab-bits boundaries; if you need byte-level access, apply the
> > +L<nbdkit-blocksize-filter(1)> before this one, to get
> > +read-modify-write access to individual bytes.
> > +
> >  =head1 PARAMETERS
> 
> I understand that the alignment of requests is enforced, but what
> happens if the client sends a request (correctly aligned) that is 17
> bytes in size, for example?
> 
> ... Aha, so is_aligned() doesn't only check "offset", it also checks
> "count". That wasn't clear to me from the addition to
> "filters/swab/nbdkit-swab-filter.pod". I suggest spelling that out more
> explicitly.

I went with:

-Note that this filter fails operations that are not aligned to the
-swab-bits boundaries; if you need byte-level access, apply the
-L<nbdkit-blocksize-filter(1)> before this one, to get
-read-modify-write access to individual bytes.
+Note that this filter fails operations where the offset or count are
+not aligned to the swab-bits boundaries; if you need byte-level
+access, apply the L<nbdkit-blocksize-filter(1)> before this one, to
+get read-modify-write access to individual bytes.

> > +/* Block size constraints. */
> > +static int
> > +swab_block_size (nbdkit_next *next, void *handle,
> > +                 uint32_t *minimum, uint32_t *preferred, uint32_t *maximum)
> > +{
> > +  if (next->block_size (next, minimum, preferred, maximum) == -1)
> > +    return -1;
> > +
> > +  if (*minimum == 0) {         /* No constraints set by the plugin. */
> > +    *minimum = bits/8;
> > +    *preferred = 512;
> > +    *maximum = 0xffffffff;
> > +  }
> > +  else {
> > +    *minimum = MAX (*minimum, bits/8);
> > +  }
> 
> Given that the count too must be a whole multiple of the swap-block size
> (correctly so), what if the underlying plugin specifies a minimum block
> size of 17?

Not possible ;) Minimum block size must be a power of 2 between 1 and
64k; the plugin layer enforces this.  Since swab-bits alignments are
1, 2, 4, or 8 (also a power of 2), the MAX() operation is sufficient
without needing ROUND_UP.

> 
> I think that will take effect here, and then this filter will specify
> such a minimum block size (17) that it will, in turn, reject
> unconditionally. That kind of defeats the purpose of exposing a "minimum
> block size".
> 
> Wouldn't it be better if, on the "else" branch, we rounded up "*minimum"?
> 
>   *minimum = ROUND_UP (*minimum, bits/8);
>

Now in as amended as commit b9f8ef83

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org