Re: [Qemu-devel] [RFC v1] block/NVMe: introduce a new vhost NVMe host device to QEMU

[Paolo Bonzini]

On 15/01/2018 09:01, Changpeng Liu wrote:
> NVMe 1.3 specification introduces a new NVMe ADMIN command:
> doorbell buffer config, which can write shadow doorbell buffer
> instead of MMIO registers, so it can improve the Guest performance
> a lot for emulated NVMe devices inside VM.
> 
> Similar with existing vhost-user-scsi solution, this commit builds a
> new vhost_user_nvme host device to VM and the I/O is processed at
> the slave I/O target, so users can implement a user space NVMe driver
> in the slave I/O target.
> 
> Users can start QEMU with: -chardev socket,id=char0,path=/path/vhost.0 \
> -device vhost-user-nvme,chardev=char0,num_io_queues=2.

Hi Changpeng,

I have two comments on this series.

First, the new command in NVMe 1.3 is great.  However, please first add
support for the doorbell buffer config in hw/block/nvme.c.  There is no
need to tie support for the new command to a completely new external
server architecture.  Emulated NVMe can be enhanced to use iothreads and
(when the doorbell buffer is configured) ioeventfd, and that should come
before enhancements for external vhost-like servers.

Second, virtio-based vhost-user remains QEMU's preferred method for
high-performance I/O in guests.  Discard support is missing and that is
important for SSDs; that should be fixed in the virtio spec.  Are there
any other features where virtio-blk is lagging behind NVMe?

Thanks,

Paolo