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

[Ferry Meng <mengferry@linux.alibaba.com>]

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On 12/5/24 5:47 AM, Stefan Hajnoczi wrote:
> On Tue, 3 Dec 2024 at 07:17, Ferry Meng <mengferry@linux.alibaba.com> 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.
> First I thought this was similar to Stefano Garzarella's previous
> virtio-blk io_uring passthrough work where a host io_uring was passed
> through into the guest:
> https://static.sched.com/hosted_files/kvmforum2020/9c/KVMForum_2020_io_uring_passthrough_Stefano_Garzarella.pdf
>
> But now I see this is a uring_cmd interface for sending virtio_blk
> commands from userspace like the one offered by the NVMe driver.
>
> Unlike NVMe, the virtio-blk command set is minimal and does not offer
> a rich set of features. Is the motivation really virtio-blk command
> passthrough or is the goal just to create a fast path for I/O?
Sure, this series only works with guest os, not between host and guest. 
Well, 'io_uring passthrough'
gives a 'fast path for I/O' , and is ideal for our use case --- like 
using virtio-blk with command directly
from userspace scales better.
> If the goal is just a fast path for I/O, then maybe Jens would
> consider a generic command set that is not device-specific? That way
> any driver (NVMe, virtio-blk, etc) can implement this uring_cmd
> interface and any application can use it without worrying about the
> underlying command set. I think a generic fast path would be much more
> useful to applications than driver-specific interfaces.
If I understand correctly, io_uring passthrough is already a complete 
abstract framework for I/O request dispatch.
Its aim is to allow driver to handle commands on its own with bypass 
unused logic. Thus I chose this method of
implementation. I really agreed that the command set of virtio-blk is 
sufficient and minimal enough, but I also believe
that this makes it more convenient for us to adapt all command types 
with fewer modifications, so that virtio-blk
can use io_uring passthrough.

Of course, we can wait for Jens' view on the preceding discussion.
>> 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(-)
>>
>> --
>> 2.43.5
>>
>>