[Qemu-block] BDRV request fragmentation and vitio-blk write submission guarantees

[Евгений Яковлев]

Hi everyone,

We're currently working on implementing a qemu BDRV format driver which 
we are using with virtio-blk devices.

I have a question concerning BDRV request fragmentation and virtio-blk 
write request submission which is not entirely clear to me by only 
reading virtio spec. Could you please consider the following case and 
give some additional guidance?

1. Our BDRV format driver has a notion of max supported transfer size. 
So we implement BlockDriver::bdrv_refresh_limits where we fill out 
BlockLimits::max_transfer and opt_transfer fields.

2. virtio-blk exposes max_transfer as a virtio_blk_config::opt_io_size 
field, which (according to spec 1.1) is a **suggested** maximum. We read 
"suggested" as "guest driver may still send requests that don't fit into 
opt_io_size and we should handle those"...

3. ... and judging by code in block/io.c qemu block layer handles such 
requests by fragmenting them into several BDRV requests if request size 
is > max_transfer

4. Guest will see request completion only after all fragments are 
handled. However each fragment submission path can call 
qemu_coroutine_yield and move on to submitting next request available in 
virtq before completely submitting the rest of the fragments. Which 
means the following situation is possible where BDRV sees 2 write 
requests in virtq, both of which are larger than max_transfer:

||

|Blocks: |-------------------------------------> Write1: xxxxxxxx 
Write2: yyyyyyyy Write1Chunk1: xxxx Write2Chunk1: yyyy Write2Chunk2: 
yyyy Write1Chunk1: xxxx Blocks: |------------yyyyxxxx----------------->|

||

|In above scenario guest virtio-blk driver decided to submit 2 
intersecting write requests, both of which are larger than 
||max_transfer, and then call hypervisor.|

|I understand that virtio-blk may handle requests out of order, so guest 
must not make any assumptions on relative order in which those requests 
will be handled.|

|However, can guest driver expect that whatever the submission order 
will be, the actual intersecting writes will be atomic?|

|In other words, will it be correct for conforming virtio-blk driver to 
expect only "|||xxxxxxxx" or "||||yyyyyyyy" but not anything else in 
between, after both requests are reported as completed?||

||Because i think that is something that may happen in qemu right now, 
if i understood correctly. ||

||||

||Thanks!| |