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Date:   Tue, 1 Mar 2022 09:06:59 +0000
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Akihiko Odaki <akihiko.odaki@gmail.com>
Cc:     linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Jens Axboe <axboe@kernel.dk>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] virtio-blk: Assign discard_granularity
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References: <20220224093802.11348-1-akihiko.odaki@gmail.com>
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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-cs=
prd01.html
> > > > discard_sector_alignment can be used by OS when splitting a request
> > > > based on alignment.
> > >=20
> > > According to Documentation/ABI/stable/sysfs-block, the corresponding
> > > field in the kernel is, confusingly, discard_granularity, not
> > > discard_alignment.
> >=20
> > Good catch, struct virtio_blk_config->discard_sector_alignment is Linux
> > q->limits.discard_granularity.
> >=20
> > >=20
> > > Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
> > > ---
> > >   drivers/block/virtio_blk.c | 4 +---
> > >   1 file changed, 1 insertion(+), 3 deletions(-)
> > >=20
> > > 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 *v=
dev)
> > >   		blk_queue_io_opt(q, blk_size * opt_io_size);
> > >   	if (virtio_has_feature(vdev, VIRTIO_BLK_F_DISCARD)) {
> > > -		q->limits.discard_granularity =3D blk_size;
> > > -
> > >   		virtio_cread(vdev, struct virtio_blk_config,
> > >   			     discard_sector_alignment, &v);
> > > -		q->limits.discard_alignment =3D v ? v << SECTOR_SHIFT : 0;
> >=20
> > 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?
>=20
> 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 cann=
ot
> test it.
>=20
> I'd like to leave this patch as is since I cannot deny the possibility th=
at
> 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@redhat.com>

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