Re: [PATCH] virtio-blk: Assign discard_granularity

[Stefan Hajnoczi]

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@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.

Fair enough. To do it properly we'd need to add a new configuration
space field to virtio-blk.

Reviewed-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>

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