Re: [Qemu-devel] [virtio-dev] [PATCH v3 0/7] Vhost-pci for inter-VM communication

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sat, Dec 09, 2017 at 04:23:17PM +0000, Wang, Wei W wrote:
> On Friday, December 8, 2017 4:34 PM, Stefan Hajnoczi wrote:
> > On Fri, Dec 8, 2017 at 6:43 AM, Wei Wang <wei.w.wang@intel.com> wrote:
> > > On 12/08/2017 07:54 AM, Michael S. Tsirkin wrote:
> > >>
> > >> On Thu, Dec 07, 2017 at 06:28:19PM +0000, Stefan Hajnoczi wrote:
> > >>>
> > >>> On Thu, Dec 7, 2017 at 5:38 PM, Michael S. Tsirkin <mst@redhat.com>
> > > Thanks Stefan and Michael for the sharing and discussion. I think
> > > above 3 and 4 are debatable (e.g. whether it is simpler really
> > > depends). 1 and 2 are implementations, I think both approaches could
> > > implement the device that way. We originally thought about one device
> > > and driver to support all types (called it transformer sometimes :-)
> > > ), that would look interesting from research point of view, but from
> > > real usage point of view, I think it would be better to have them separated,
> > because:
> > > - different device types have different driver logic, mixing them
> > > together would cause the driver to look messy. Imagine that a
> > > networking driver developer has to go over the block related code to
> > > debug, that also increases the difficulty.
> > 
> > I'm not sure I understand where things get messy because:
> > 1. The vhost-pci device implementation in QEMU relays messages but has no
> > device logic, so device-specific messages like VHOST_USER_NET_SET_MTU are
> > trivial at this layer.
> > 2. vhost-user slaves only handle certain vhost-user protocol messages.
> > They handle device-specific messages for their device type only.  This is like
> > vhost drivers today where the ioctl() function returns an error if the ioctl is
> > not supported by the device.  It's not messy.
> > 
> > Where are you worried about messy driver logic?
> 
> Probably I didn’t explain well, please let me summarize my thought a little bit, from the perspective of the control path and data path.
> 
> Control path: the vhost-user messages - I would prefer just have the interaction between QEMUs, instead of relaying to the GuestSlave, because
> 1) I think the claimed advantage (easier to debug and develop) doesn’t seem very convincing

You are defining a mapping from the vhost-user protocol to a custom
virtio device interface.  Every time the vhost-user protocol (feature
bits, messages, etc) is extended it will be necessary to map this new
extension to the virtio device interface.

That's non-trivial.  Mistakes are possible when designing the mapping.
Using the vhost-user protocol as the device interface minimizes the
effort and risk of mistakes because most messages are relayed 1:1.

> 2) some messages can be directly answered by QemuSlave , and some messages are not useful to give to the GuestSlave (inside the VM), e.g. fds, VhostUserMemoryRegion from SET_MEM_TABLE msg (the device first maps the master memory and gives the offset (in terms of the bar, i.e., where does it sit in the bar) of the mapped gpa to the guest. if we give the raw VhostUserMemoryRegion to the guest, that wouldn’t be usable).

I agree that QEMU has to handle some of messages, but it should still
relay all (possibly modified) messages to the guest.

The point of using the vhost-user protocol is not just to use a familiar
binary encoding, it's to match the semantics of vhost-user 100%.  That
way the vhost-user software stack can work either in host userspace or
with vhost-pci without significant changes.

Using the vhost-user protocol as the device interface doesn't seem any
harder than defining a completely new virtio device interface.  It has
the advantages that I've pointed out:

1. Simple 1:1 mapping for most that is easy to maintain as the
   vhost-user protocol grows.

2. Compatible with vhost-user so slaves can run in host userspace
   or the guest.

I don't see why it makes sense to define new device interfaces for each
device type and create a software stack that is incompatible with
vhost-user.

> 
> 
> Data path: that's the discussion we had about one driver or separate driver for different device types, and this is not related to the control path.
> I meant if we have one driver for all the types, that driver would look messy, because each type has its own data sending/receiving logic. For example, net type deals with a pair of tx and rx, and transmission is skb based (e.g. xmit_skb), while block type deals with a request queue. If we have one driver, then the driver will include all the things together.

I don't understand this.  Why would we have to put all devices (net,
scsi, etc) into just one driver?  The device drivers sit on top of the
vhost-pci driver.

For example, imagine a libvhost-user application that handles the net
device.  The vhost-pci vfio driver would be part of libvhost-user and
the application would only emulate the net device (RX and TX queues).

Stefan

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