Re: [PATCH 0/3][RFC] virtio-blk: add io_uring passthrough support for virtio-blk

Ferry Meng
2024-12-16T12:07:14Z

On 12/16/24 3:38 PM, Jason Wang wrote:
> On Mon, Dec 16, 2024 at 10:01 AM Ferry Meng <mengferry@linux.alibaba.com> wrote:
>>
>> On 12/3/24 8:14 PM, Ferry Meng wrote:
>>> We seek to develop a more flexible way to use virtio-blk and bypass the block
>>> layer logic in order to accomplish certain performance optimizations. As a
>>> result, we referred to the implementation of io_uring passthrough in NVMe
>>> and implemented it in the virtio-blk driver. This patch series adds io_uring
>>> passthrough support for virtio-blk devices, resulting in lower submit latency
>>> and increased flexibility when utilizing virtio-blk.
>>>
>>> To test this patch series, I changed fio's code:
>>> 1. Added virtio-blk support to engines/io_uring.c.
>>> 2. Added virtio-blk support to the t/io_uring.c testing tool.
>>> Link: https://github.com/jdmfr/fio
>>>
>>> Using t/io_uring-vblk, the performance of virtio-blk based on uring-cmd
>>> scales better than block device access. (such as below, Virtio-Blk with QEMU,
>>> 1-depth fio)
>>> (passthru) read: IOPS=17.2k, BW=67.4MiB/s (70.6MB/s)
>>> slat (nsec): min=2907, max=43592, avg=3981.87, stdev=595.10
>>> clat (usec): min=38, max=285,avg=53.47, stdev= 8.28
>>> lat (usec): min=44, max=288, avg=57.45, stdev= 8.28
>>> (block) read: IOPS=15.3k, BW=59.8MiB/s (62.7MB/s)
>>> slat (nsec): min=3408, max=35366, avg=5102.17, stdev=790.79
>>> clat (usec): min=35, max=343, avg=59.63, stdev=10.26
>>> lat (usec): min=43, max=349, avg=64.73, stdev=10.21
>>>
>>> Testing the virtio-blk device with fio using 'engines=io_uring_cmd'
>>> and 'engines=io_uring' also demonstrates improvements in submit latency.
>>> (passthru) taskset -c 0 t/io_uring-vblk -b4096 -d8 -c4 -s4 -p0 -F1 -B0 -O0 -n1 -u1 /dev/vdcc0
>>> IOPS=189.80K, BW=741MiB/s, IOS/call=4/3
>>> IOPS=187.68K, BW=733MiB/s, IOS/call=4/3
>>> (block) taskset -c 0 t/io_uring-vblk -b4096 -d8 -c4 -s4 -p0 -F1 -B0 -O0 -n1 -u0 /dev/vdc
>>> IOPS=101.51K, BW=396MiB/s, IOS/call=4/3
>>> IOPS=100.01K, BW=390MiB/s, IOS/call=4/4
>>>
>>> The performance overhead of submitting IO can be decreased by 25% overall
>>> with this patch series. The implementation primarily references 'nvme io_uring
>>> passthrough', supporting io_uring_cmd through a separate character interface
>>> (temporarily named /dev/vdXc0). Since this is an early version, many
>>> details need to be taken into account and redesigned, like:
>>> ● Currently, it only considers READ/WRITE scenarios, some more complex operations
>>> not included like discard or zone ops.(Normal sqe64 is sufficient, in my opinion;
>>> following upgrades, sqe128 and cqe32 might not be needed).
>>> ● ......
>>>
>>> I would appreciate any useful recommendations.
>>>
>>> Ferry Meng (3):
>>> virtio-blk: add virtio-blk chardev support.
>>> virtio-blk: add uring_cmd support for I/O passthru on chardev.
>>> virtio-blk: add uring_cmd iopoll support.
>>>
>>> drivers/block/virtio_blk.c | 325 +++++++++++++++++++++++++++++++-
>>> include/uapi/linux/virtio_blk.h | 16 ++
>>> 2 files changed, 336 insertions(+), 5 deletions(-)
>> Hi, Micheal & Jason :
>>
>> What about yours' opinion? As virtio-blk maintainer. Looking forward to
>> your reply.
>>
>> Thanks
> If I understand this correctly, this proposal wants to make io_uring a
> transport of the virito-blk command. So the application doesn't need
> to worry about compatibility etc. This seems to be fine.
>
> But I wonder what's the security consideration, for example do we
> allow all virtio-blk commands to be passthroughs and why.

About 'security consideration', the generic char-dev belongs to root, so
only root can use this passthrough path.

On the other hand, to what I know, virtio-blk commands are all related
to 'I/O operations', so we can support all those opcodes with bypassing
vfs&block layer (if we want). I just realized the most  basic read/write
in this RFC patch series, others will be considered later.

> Thanks
>