On Tue, Mar 01, 2022 at 02:43:55PM +0900, Akihiko Odaki
wrote:> On 2022/02/28 19:51, Stefan Hajnoczi wrote:
> > On Thu, Feb 24, 2022 at 06:38:02PM +0900, Akihiko Odaki wrote:
> > > Virtual I/O Device (VIRTIO) Version 1.1
> > >
https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html
> > > > discard_sector_alignment can be used by OS when splitting a
request
> > > > based on alignment.
> > >
> > > According to Documentation/ABI/stable/sysfs-block, the
corresponding
> > > field in the kernel is, confusingly, discard_granularity, not
> > > discard_alignment.
> >
> > Good catch, struct virtio_blk_config->discard_sector_alignment is
Linux
> > q->limits.discard_granularity.
> >
> > >
> > > Signed-off-by: Akihiko Odaki <akihiko.odaki at gmail.com>
> > > ---
> > > drivers/block/virtio_blk.c | 4 +---
> > > 1 file changed, 1 insertion(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/block/virtio_blk.c
b/drivers/block/virtio_blk.c
> > > index c443cd64fc9b..1fb3c89900e3 100644
> > > --- a/drivers/block/virtio_blk.c
> > > +++ b/drivers/block/virtio_blk.c
> > > @@ -913,11 +913,9 @@ static int virtblk_probe(struct
virtio_device *vdev)
> > > blk_queue_io_opt(q, blk_size * opt_io_size);
> > > if (virtio_has_feature(vdev, VIRTIO_BLK_F_DISCARD)) {
> > > - q->limits.discard_granularity = blk_size;
> > > -
> > > virtio_cread(vdev, struct virtio_blk_config,
> > > discard_sector_alignment, &v);
> > > - q->limits.discard_alignment = v ? v << SECTOR_SHIFT :
0;
> >
> > Should we use struct virtio_blk_config->topology.alignment_offset
> > ("offset of first aligned logical block" and used for Linux
> > blk_queue_alignment_offset()) for q->limits.discard_alignment?
>
> Maybe but I'm not sure. I had looked at the code of QEMU
> (commit 5c1ee569660d4a205dced9cb4d0306b907fb7599) but it apparently always
> sets 0 for virtio_blk_config->topology.alignment_offset.
> I don't have a hardware which requires discard_alignment either so I
cannot
> test it.
>
> I'd like to leave this patch as is since I cannot deny the possibility
that
> the host has a different alignment offset for discarding and other
> operations.
Fair enough. To do it properly we'd need to add a new configuration
space field to virtio-blk.
Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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