Re: [PATCH 4/4] block: introduce BDRV_MAX_LENGTH

["Richard W.M. Jones" <rjones@redhat.com>]

On Thu, Jan 07, 2021 at 09:58:17AM +0000, Richard W.M. Jones wrote:
> On Fri, Dec 04, 2020 at 01:27:13AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > Finally to be safe with calculations, to not calculate different
> > maximums for different nodes (depending on cluster size and
> > request_alignment), let's simply set QEMU_ALIGN_DOWN(INT64_MAX, 2^30)
> > as absolute maximum bytes length for Qemu. Actually, it's not much less
> > than INT64_MAX.
> 
> > +/*
> > + * We want allow aligning requests and disk length up to any 32bit alignment
> > + * and don't afraid of overflow.
> > + * To achieve it, and in the same time use some pretty number as maximum disk
> > + * size, let's define maximum "length" (a limit for any offset/bytes request and
> > + * for disk size) to be the greatest power of 2 less than INT64_MAX.
> > + */
> > +#define BDRV_MAX_ALIGNMENT (1L << 30)
> > +#define BDRV_MAX_LENGTH (QEMU_ALIGN_DOWN(INT64_MAX, BDRV_MAX_ALIGNMENT))
> 
> This change broke nbdkit tests.

Actually that's not the only problem.  It appears that we're unable to
read or write the last sector of this disk:

$ nbdkit memory $(( 2**63 - 2**30 )) --run 'build/qemu-io -r -f raw "$uri" -c "r -v $(( 2**63 - 2**30 - 512 )) 512" ' 
read failed: Input/output error

$ nbdkit memory $(( 2**63 - 2**30 )) --run 'build/qemu-io -f raw "$uri" -c "w -P 3 $(( 2**63 - 2**30 - 512 )) 512" ' 
write failed: Input/output error

You can play around with the constants.  I found it's possible to read
and write the non-aligned 512 bytes starting at 2^63-2^30-513.  Could
be a fencepost error somewhere in qemu?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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