Re: [Qemu-block] [Qemu-devel] [PATCH 2/8] qcow: Switch get_cluster_offset to be byte-based

[Eric Blake]

On 05/28/2018 05:52 AM, Kevin Wolf wrote:
> Am 25.04.2018 um 20:32 hat Eric Blake geschrieben:
> > We are gradually moving away from sector-based interfaces, towards
> > byte-based.  Make the change for the internal helper function
> > get_cluster_offset(), by changing n_start and n_end to by byte
> > offsets rather than sector indices within the cluster being
> > allocated.
> >
> > A later patch will then switch the qcow driver as a whole over
> > to byte-based operation.
> >
> > Signed-off-by: Eric Blake <address@hidden>
> > ---
> >   block/qcow.c | 28 ++++++++++++++--------------
> >   1 file changed, 14 insertions(+), 14 deletions(-)

> > +                    for (i = 0; i < s->cluster_size; i += BDRV_SECTOR_SIZE) {
> >                           if (i < n_start || i >= n_end) {
> > -                            memset(s->cluster_data, 0x00, 512);
> > +                            memset(s->cluster_data, 0x00, BDRV_SECTOR_SIZE);
> >                               if (qcrypto_block_encrypt(s->crypto,
> > -                                                      (start_sect + i) *
> > -                                                      BDRV_SECTOR_SIZE,
> > +                                                      start_offset + i,
> >                                                         s->cluster_data,
> >                                                         BDRV_SECTOR_SIZE,
> >                                                         NULL) < 0) {
>
> This code is still working in blocks of BDRV_SECTOR_SIZE here - which
> you do need to keep at least partially because that's the block size
> that qcrypto_block_encrypt() works with. qcrypto_block_qcow_encrypt()
> even asserts it.
>
> However, this means that even though n_start and n_end are byte-based
> now, the code only works correctly with encrypted images if they are
> multiples of BDRV_SECTOR_SIZE. This is currently true and we could
> assert it, but it would kind of defeat the purpose of the patch.

But in patch 5, I intentionally kept bs->bl.request_alignment at 512, so 
I'd rather just assert that n_start and n_end are properly aligned than 
to worry about rounding issues.

> I suppose you could make unaligned n_start/n_end work if you round down
> n_start and round up n_end to the next sector boundary for the
> comparison with i. For unaligned requests, we would then write a bit
> more than is actually necessary, but I think that's okay because we're
> initialising a previously unallocated cluster, so we don't overwrite
> valid data.

The point is that we never have unaligned requests to qcow1.

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