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[Qemu-devel] BDRV request fragmentation and virtio-blk write submission

From: Евгений Яковлев
Subject: [Qemu-devel] BDRV request fragmentation and virtio-blk write submission guarantees (2nd try)
Date: Thu, 18 Jul 2019 16:52:46 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

Hi everyone,

My previous message was misformatted, so here's another one. Sorry about that.

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.


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