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Subject: Re: [virtio-dev] Re: [PATCH v2 1/4] virtio-blk: document data[] size constraints
On Mon, Feb 18, 2019 at 02:04:20PM +0000, Stefan Hajnoczi wrote: > On Mon, Feb 18, 2019 at 08:22:00AM +0100, Jan Kiszka wrote: > > 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. > > > > + > > > > > I'm not the original spec author. Feel free to correct me if this is > wrong: > > > > 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? > > The multiple segments feature was included because the underlying > storage might support it. Currently we don't expect much use but future > hardware may rely on it more heavily (e.g. for performance). > > > > And what's the value of status > > > in case some of the requests fail? > > A failure for any segment causes the entire request to fail with no > information about which segments completed or failed. And what's the status? > > 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. > > With #32 applied the spec says: > > "max_discard_seg can be read to determine the [...] maximum number of discard segments for the block driver to use" > > and > > "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." > > This is not very explicit but it means multiple struct > virtio_blk_discard_write_zeroes can be included in a request, up to > max_discard_seg. > > I think two things are appropriate: > 1. A driver normative statement saying up to > max_discard_seg/max_write_zeroes_seg structs may be included in a > request > 2. A general description that says DISCARD/WRITE_ZEROES requests may > have more than 1 "segment" (struct virtio_blk_discard_write_zeroes) > > Does that sound good? > > Stefan Also pls include explanation about failure mode. -- MST
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