Re: [virtio-dev] Re: [PATCH v2 1/4] virtio-blk: document data[] size constraints

[Jan Kiszka <jan.kiszka@siemens.com>]

On 31.01.19 05:14, Michael S. Tsirkin wrote:
> On Thu, Jan 31, 2019 at 10:36:14AM +0800, Stefan Hajnoczi wrote:
> > The struct virtio_blk_req->data[] field is a multiple of 512 bytes long
> > for read and write requests.  Flush requests don't use data[] at all.
> >
> > The new discard and write zeroes requests being introduced in VIRTIO 1.1
> > put struct virtio_blk_discard_write_zeroes elements into data[], so it
> > must be a multiple of the struct size.
> >
> > The uint8_t data[][512] pseudo-code makes it look like discard and write
> > zeroes requests must pad to 512 bytes.  This wastes memory since struct
> > virtio_blk_discard_write_data is only 16 bytes long.
> >
> > Furthermore, all known implementations wishing to take advantage of this
> > upcoming VIRTIO 1.1 feature do not use 512-byte padding (Linux
> > virtio_blk.ko, QEMU virtio-blk device emulation, the SPDK virtio-blk
> > driver, and the SPDK vhost-user-blk device backend).
> >
> > This patch documents the data[] size constraints clearly in the driver
> > normative section.  This is clearer than the current pseudo-code.
> >
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Changpeng Liu <changpeng.liu@intel.com>
> > Cc: Stefano Garzarella <sgarzare@redhat.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >   content.tex | 14 +++++++++++++-
> >   1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/content.tex b/content.tex
> > index 836ee52..b185bb0 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -3941,7 +3941,7 @@ struct virtio_blk_req {
> >           le32 type;
> >           le32 reserved;
> >           le64 sector;
> > -        u8 data[][512];
> > +        u8 data[];
> >           u8 status;
> >   };
> >   
> > @@ -3971,6 +3971,11 @@ The \field{sector} number indicates the offset (multiplied by 512) where
> >   the read or write is to occur. This field is unused and set to 0 for
> >   commands other than read or write.
> >   
> > +VIRTIO_BLK_T_IN requests populate \field{data} with the contents of sectors
> > +read from the block device (in multiples of 512 bytes).  VIRTIO_BLK_T_OUT
> > +requests write the contents of \field{data} to the block device (in multiples
> > +of 512 bytes).
> > +
> >   The \field{data} used for discard or write zeroes command is described
> >   by one or more virtio_blk_discard_write_zeroes structs. \field{sector}
> >   indicates the starting offset (in 512-byte units) of the segment, while
> > @@ -3997,6 +4002,13 @@ A driver SHOULD accept the VIRTIO_BLK_F_RO feature if offered.
> >   A driver MUST set \field{sector} to 0 for a VIRTIO_BLK_T_FLUSH request.
> >   A driver SHOULD NOT include any data in a VIRTIO_BLK_T_FLUSH request.
> >   
> > +The length of \field{data} MUST be a multiple of 512 bytes for VIRTIO_BLK_T_IN
> > +and VIRTIO_BLK_T_OUT requests.
> > +
> > +The length of \field{data} MUST be a multiple of the size of struct
> > +virtio_blk_discard_write_zeroes for VIRTIO_BLK_T_DISCARD and
> > +VIRTIO_BLK_T_WRITE_ZEROES requests.
> > +
>
> So a single request can discard/write multiple ranges?
> It might be a good idea to make this explicit.
> Also is this capability useful/used? And what's the value of status
> in case some of the requests fail?

What happened with this comment? I don't see a follow-up nor a resolution 
elsewhere, just the opening of issue #32 for voting. Please clarify.

Jan

> >   If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the driver MAY
> >   switch to writethrough or writeback mode by writing respectively 0 and
> >   1 to the \field{writeback} field.  After writing a 0 to \field{writeback},
> > --
> > 2.20.1
>
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