Re: [PATCH] virtio-blk: Assign discard_granularity

[Akihiko Odaki]

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@xxxxxxxxx>
> > ---
> >   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.