[RFC PATCH 0/2] virtio nvme

[Nicholas A. Bellinger]

On Fri, 2015-09-18 at 11:12 -0700, Ming Lin wrote:
> On Thu, 2015-09-17 at 17:55 -0700, Nicholas A. Bellinger wrote:
> > On Thu, 2015-09-17 at 16:31 -0700, Ming Lin wrote:
> > > On Wed, 2015-09-16 at 23:10 -0700, Nicholas A. Bellinger wrote:
> > > > Hi Ming & Co,

<SNIP>

> > > > > I think the future "LIO NVMe target" only speaks NVMe protocol.
> > > > > 
> > > > > Nick(CCed), could you correct me if I'm wrong?
> > > > > 
> > > > > For SCSI stack, we have:
> > > > > virtio-scsi(guest)
> > > > > tcm_vhost(or vhost_scsi, host)
> > > > > LIO-scsi-target
> > > > > 
> > > > > For NVMe stack, we'll have similar components:
> > > > > virtio-nvme(guest)
> > > > > vhost_nvme(host)
> > > > > LIO-NVMe-target
> > > > > 
> > > > 
> > > > I think it's more interesting to consider a 'vhost style' driver that
> > > > can be used with unmodified nvme host OS drivers.
> > > > 
> > > > Dr. Hannes (CC'ed) had done something like this for megasas a few years
> > > > back using specialized QEMU emulation + eventfd based LIO fabric driver,
> > > > and got it working with Linux + MSFT guests.
> > > > 
> > > > Doing something similar for nvme would (potentially) be on par with
> > > > current virtio-scsi+vhost-scsi small-block performance for scsi-mq
> > > > guests, without the extra burden of a new command set specific virtio
> > > > driver.
> > > 
> > > Trying to understand it.
> > > Is it like below?
> > > 
> > >   .------------------------.   MMIO   .---------------------------------------.
> > >   | Guest                  |--------> | Qemu                                  |
> > >   | Unmodified NVMe driver |<-------- | NVMe device simulation(eventfd based) |
> > >   '------------------------'          '---------------------------------------'
> > >                                                   |          ^
> > >                                       write NVMe  |          |  notify command
> > >                                       command     |          |  completion
> > >                                       to eventfd  |          |  to eventfd
> > >                                                   v          |
> > >                                       .--------------------------------------.
> > >                                       | Host:                                |
> > >                                       | eventfd based LIO NVMe fabric driver |
> > >                                       '--------------------------------------'
> > >                                                         |
> > >                                                         | nvme_queue_rq()
> > >                                                         v
> > >                                        .--------------------------------------.
> > >                                        | NVMe driver                          |
> > >                                        '--------------------------------------'
> > >                                                         |
> > >                                                         |
> > >                                                         v
> > >                                        .-------------------------------------.
> > >                                        | NVMe device                         |
> > >                                        '-------------------------------------'
> > > 
> > 
> > Correct.  The LIO driver on KVM host would be handling some amount of
> > NVMe host interface emulation in kernel code, and would be able to
> > decode nvme Read/Write/Flush operations and translate -> submit to
> > existing backend drivers.
> 
> Let me call the "eventfd based LIO NVMe fabric driver" as
> "tcm_eventfd_nvme"
> 
> Currently, LIO frontend driver(iscsi, fc, vhost-scsi etc) talk to LIO
> backend driver(fileio, iblock etc) with SCSI commands.
> 
> Did you mean the "tcm_eventfd_nvme" driver need to translate NVMe
> commands to SCSI commands and then submit to backend driver?
> 

IBLOCK + FILEIO + RD_MCP don't speak SCSI, they simply process I/Os with
LBA + length based on SGL memory or pass along a FLUSH with LBA +
length.

So once the 'tcm_eventfd_nvme' driver on KVM host receives a nvme host
hardware frame via eventfd, it would decode the frame and send along the
Read/Write/Flush when exposing existing (non nvme native) backend
drivers.

This doesn't apply to PSCSI backend driver of course, because it expects
to process actual SCSI CDBs.

> But I thought the future "LIO NVMe target" can support frontend driver
> talk to backend driver directly with NVMe commands without translation.
> 

The native target_core_nvme backend driver is not processing nvme
commands per-say, but simply exposing nvme hardware queue resources to a
frontend fabric driver.

For the nvme-over-fabrics case, backend nvme submission/complete queues
are mapped to RDMA hardware queues.  So essentially the nvme physical
region page (PRP) is mapped to ib_sgl->addr.

For a 'tcm_eventfd_nvme' style host-paravirt fabric case, it's less
clear how exposing native nvme backend hardware resources would work, or
if there is a significant performance benefit over just using
submit_bio() for Read/Write/Flush.

--nab