Re: [dpdk-dev] [PATCH 00/28] vhost: add virtio-vhost-user transport

[Nikos Dragazis <ndragazis@arrikto.com>]

On 20/6/19 2:35 μ.μ., Maxime Coquelin wrote:
> Hi Nikos,
>
> On 6/19/19 5:14 PM, Nikos Dragazis wrote:
>> Hi everyone,
>>
>> this patch series introduces the concept of the virtio-vhost-user
>> transport. This is actually a revised version of an earlier RFC
>> implementation that has been proposed by Stefan Hajnoczi [1]. Though
>> this is a great feature, it seems to have been stalled, so I’d like to
>> restart the conversation on this and hopefully get it merged with your
>> help. Let me give you an overview.
>
> Thanks for taking over the series!
>
> I think you are already aware of that, but it arrives too late to
> consider it for v19.08, as the proposal deadline is over by almost 3
> weeks.
>
> That said, it is good that you sent it early, so that we can work to
> make it in for v19.11.

That's totally fine.

>
>> The virtio-vhost-user transport is a vhost-user transport implementation
>> that is based on the virtio-vhost-user device. Its key difference with
>> the existing transport is that it allows deploying vhost-user targets
>> inside dedicated Storage Appliance VMs instead of host user space. In
>> other words, it allows having guests that act as vhost-user backends for
>> other guests.
>>
>> The virtio-vhost-user device implements the vhost-user control plane
>> (master-slave communication) as follows:
>>
>> 1. it parses the vhost-user messages from the vhost-user unix domain
>>     socket and forwards them to the slave guest through virtqueues
>>
>> 2. it maps the vhost memory regions in QEMU’s process address space and
>>     exposes them to the slave guest as a RAM-backed PCI MMIO region
>>
>> 3. it hooks up doorbells to the callfds. The slave guest can use these
>>     doorbells to interrupt the master guest driver
>>
>> The device code has not yet been merged into upstream QEMU, but this is
>> definitely the end goal.
>
> Could you provide a pointer to the QEMU series, and instructions to test
> this new device?

Of course. Please have a look at the following step-by-step guide:

https://github.com/ndragazis/ndragazis.github.io/blob/master/dpdk-vhost-vvu-demo.md

If you face any problems or you find that something breaks (I hope not
:) ) please let me know. I haven't done thorough testing so I may have
missed something.  Please, also bear in mind that the device code is not
finished. There are still some things that need to be done.

>
>> The current state is that we are awaiting for
>> the approval of the virtio spec.
>
> Ditto, a link to the spec patches would be useful.

You can find the patches here:

https://lists.oasis-open.org/archives/virtio-dev/201905/msg00022.html

and an HTML version is available here:

https://ndragazis.github.io/virtio-v1.1-wd02.html#x1-41000011

>
>> I have Cced Darek from the SPDK community who has helped me a lot by
>> reviewing this series. Note that any device type could be implemented
>> over this new transport. So, adding the virtio-vhost-user transport in
>> DPDK would allow using it from SPDK as well.
>>
>> Getting into the code internals, this patch series makes the following
>> changes:
>>
>> 1. introduce a generic interface for the transport-specific operations.
>>     Each of the two available transports, the pre-existing AF_UNIX
>>     transport and the virtio-vhost-user transport, is going to implement
>>     this interface. The AF_UNIX-specific code has been extracted from the
>>     core vhost-user code and is now part of the AF_UNIX transport
>>     implementation in trans_af_unix.c.
>>
>> 2. introduce the virtio-vhost-user transport. The virtio-vhost-user
>>     transport requires a driver for the virtio-vhost-user devices. The
>>     driver along with the transport implementation have been packed into
>>     a separate library in `drivers/virtio_vhost_user/`. The necessary
>>     virtio-pci code has been copied from `drivers/net/virtio/`. Some
>>     additional changes have been made so that the driver can utilize the
>>     additional resources of the virtio-vhost-user device.
>>
>> 3. update librte_vhost public API to enable choosing transport for each
>>     new vhost device. Extend the vhost net driver and vhost-scsi example
>>     application to export this new API to the end user.
>>
>> The primary changes I did to Stefan’s RFC implementation are the
>> following:
>>
>> 1. moved postcopy live migration code into trans_af_unix.c. Postcopy
>>     live migration relies on the userfault fd mechanism, which cannot be
>>     supported by virtio-vhost-user.
>>
>> 2. moved setup of the log memory region into trans_af_unix.c. Setting up
>>     the log memory region involves mapping/unmapping guest memory. This
>>     is an AF_UNIX transport-specific operation.
>>
>> 3. introduced a vhost transport operation for
>>     process_slave_message_reply()
>>
>> 4. moved the virtio-vhost-user transport/driver into a separate library
>>     in `drivers/virtio_vhost_user/`. This required making vhost.h and
>>     vhost_user.h part of librte_vhost public API and exporting some
>>     private symbols via the version script. This looks better to me that
>>     just moving the entire librte_vhost into `drivers/`. I am not sure if
>>     this is the most appropriate solution. I am looking forward to your
>>     suggestions on this.
>
> I'm not sure this is the right place to put it.

Okay. Is there something specific that you think that doesn't fit
nicely? Do you have something else in mind?

>
>> 5. made use of the virtio PCI capabilities for the additional device
>>     resources (doorbells, shared memory). This required changes in
>>     virtio_pci.c and trans_virtio_vhost_user.c.
>>
>> 6. [minor] changed some commit headlines to comply with
>>     check-git-log.sh.
>>
>> Please, have a look and let me know about your thoughts. Any
>> reviews/pointers/suggestions are welcome.
>
> Maxime