Re: [Qemu-devel] [PATCH v6 0/8] Vhost and vhost-net support for userspace based backends

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 14, 2014 at 07:13:43PM +0100, Antonios Motakis wrote:
> 
> 
> 
> On Tue, Jan 14, 2014 at 12:33 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>     On Mon, Jan 13, 2014 at 03:25:11PM +0100, Antonios Motakis wrote:
>     > In this patch series we would like to introduce our approach for putting
>     a
>     > virtio-net backend in an external userspace process. Our eventual target
>     is to
>     > run the network backend in the Snabbswitch ethernet switch, while
>     receiving
>     > traffic from a guest inside QEMU/KVM which runs an unmodified virtio-net
>     > implementation.
>     >
>     > For this, we are working into extending vhost to allow equivalent
>     functionality
>     > for userspace. Vhost already passes control of the data plane of
>     virtio-net to
>     > the host kernel; we want to realize a similar model, but for userspace.
>     >
>     > In this patch series the concept of a vhost-backend is introduced.
>     >
>     > We define two vhost backend types - vhost-kernel and vhost-user. The
>     former is
>     > the interface to the current kernel module implementation. Its control
>     plane is
>     > ioctl based. The data plane is the kernel directly accessing the QEMU
>     allocated,
>     > guest memory.
>     >
>     > In the new vhost-user backend, the control plane is based on
>     communication
>     > between QEMU and another userspace process using a unix domain socket.
>     This
>     > allows to implement a virtio backend for a guest running in QEMU, inside
>     the
>     > other userspace process.
>     >
>     > We change -mem-path to QemuOpts and add prealloc, share and unlink as
>     properties
>     > to it. HugeTLBFS requirements of -mem-path are relaxed, so any valid path
>     can
>     > be used now. The new properties allow more fine grained control over the
>     guest
>     > RAM backing store.
>     >
>     > The data path is realized by directly accessing the vrings and the buffer
>     data
>     > off the guest's memory.
>     >
>     > The current user of vhost-user is only vhost-net. We add new netdev
>     backend
>     > that is intended to initialize vhost-net with vhost-user backend.
> 
>     Some meta comments.
> 
>     Something that makes this patch harder to review is how it's
>     split up. Generally IMHO it's not a good idea to repeatedly
>     edit same part of file adding stuff in patch after patch,
>     it's only making things harder to read if you add stubs, then fill them up.
>     (we do this sometimes when we are changing existing code, but
>     it is generally not needed when adding new code)
> 
>     Instead, split it like this:
> 
>     1. general refactoring, split out linux specific and generic parts
>        and add the ops indirection
>     2. add new files for vhost-user with complete implementation.
>        without command line to support it, there will be no way to use it,
>        but should build fine.
>     3. tie it all up with option parsing
> 
> 
>     Generic vhost and vhost net files should be kept separate.
>     Don't let vhost net stuff seep back into generic files,
>     we have vhost-scsi too.
>     I would also prefer that userspace vhost has its own files.
> 
> 
> Ok, we'll keep this into account.
>  
> 
> 
>     We need a small test server qemu can talk to, to verify things
>     actually work.
> 
> 
> We have implemented such a test app: https://github.com/virtualopensystems/vapp
> 
> We use it for testing, and also as a reference implementation. A client is also
> included.
> 

Sounds good. Can we include this in qemu and tie
it into the qtest framework?
>From a brief look, it merely needs to be tweaked for portability,
unless 

> 
>     Already commented on: reuse the chardev syntax and preferably code.
>     We already support a bunch of options there for
>     domain sockets that will be useful here, they should
>     work here as well.
> 
> 
> We adapted the syntax for this to be consistent with chardev. What we didn't
> use, it is not obvious at all to us on how they should be used; a lot of the
> chardev options just don't apply to us.
>  

Well server option should work at least.
nowait can work too?

Also, if reconnect is useful it should be for chardevs too, so if we don't
share code, need to code it in two places to stay consistent.

Overall sharing some code might be better ...

>     In particular you shouldn't require filesystem access by qemu,
>     passing fd for domain socket should work.
> 
> 
> We can add an option to pass an fd for the domain socket if needed. However as
> far as we understand, chardev doesn't do that either (at least form looking at
> the man page). Maybe we misunderstand what you mean.

Sorry. I got confused with e.g. tap which has this. This might be
useful but does not have to block this patch.

> 
> 
>     > Example usage:
>     >
>     > qemu -m 1024 -mem-path /hugetlbfs,prealloc=on,share=on \
>     >      -netdev type=vhost-user,id=net0,path=/path/to/sock,poll_time=2500 \
>     >      -device virtio-net-pci,netdev=net0
> 
>     It's not clear which parts of -mem-path are required for vhost-user.
>     It should be documented somewhere, made clear in -help
>     and should fail gracefully if misconfigured.
> 
> 
> 
> Ok.
>  
> 
> 
>     >
>     > Changes from v5:
>     >  - Split -mem-path unlink option to a separate patch
>     >  - Fds are passed only in the ancillary data
>     >  - Stricter message size checks on receive/send
>     >  - Netdev vhost-user now includes path and poll_time options
>     >  - The connection probing interval is configurable
>     >
>     > Changes from v4:
>     >  - Use error_report for errors
>     >  - VhostUserMsg has new field `size` indicating the following payload
>     length.
>     >    Field `flags` now has version and reply bits. The structure is packed.
>     >  - Send data is of variable length (`size` field in message)
>     >  - Receive in 2 steps, header and payload
>     >  - Add new message type VHOST_USER_ECHO, to check connection status
>     >
>     > Changes from v3:
>     >  - Convert -mem-path to QemuOpts with prealloc, share and unlink
>     properties
>     >  - Set 1 sec timeout when read/write to the unix domain socket
>     >  - Fix file descriptor leak
>     >
>     > Changes from v2:
>     >  - Reconnect when the backend disappears
>     >
>     > Changes from v1:
>     >  - Implementation of vhost-user netdev backend
>     >  - Code improvements
>     >
>     > Antonios Motakis (8):
>     >   Convert -mem-path to QemuOpts and add prealloc and share properties
>     >   New -mem-path option - unlink.
>     >   Decouple vhost from kernel interface
>     >   Add vhost-user skeleton
>     >   Add domain socket communication for vhost-user backend
>     >   Add vhost-user calls implementation
>     >   Add new vhost-user netdev backend
>     >   Add vhost-user reconnection
>     >
>     >  exec.c                            |  57 +++-
>     >  hmp-commands.hx                   |   4 +-
>     >  hw/net/vhost_net.c                | 144 +++++++---
>     >  hw/net/virtio-net.c               |  42 ++-
>     >  hw/scsi/vhost-scsi.c              |  13 +-
>     >  hw/virtio/Makefile.objs           |   2 +-
>     >  hw/virtio/vhost-backend.c         | 556
>     ++++++++++++++++++++++++++++++++++++++
>     >  hw/virtio/vhost.c                 |  46 ++--
>     >  include/exec/cpu-all.h            |   3 -
>     >  include/hw/virtio/vhost-backend.h |  40 +++
>     >  include/hw/virtio/vhost.h         |   4 +-
>     >  include/net/vhost-user.h          |  17 ++
>     >  include/net/vhost_net.h           |  15 +-
>     >  net/Makefile.objs                 |   2 +-
>     >  net/clients.h                     |   3 +
>     >  net/hub.c                         |   1 +
>     >  net/net.c                         |   2 +
>     >  net/tap.c                         |  16 +-
>     >  net/vhost-user.c                  | 177 ++++++++++++
>     >  qapi-schema.json                  |  21 +-
>     >  qemu-options.hx                   |  24 +-
>     >  vl.c                              |  41 ++-
>     >  22 files changed, 1106 insertions(+), 124 deletions(-)
>     >  create mode 100644 hw/virtio/vhost-backend.c
>     >  create mode 100644 include/hw/virtio/vhost-backend.h
>     >  create mode 100644 include/net/vhost-user.h
>     >  create mode 100644 net/vhost-user.c
>     >
>     > --
>     > 1.8.3.2
>     >
> 
>