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 Thu, Dec 14, 2017 at 01:53:16PM +0800, Wei Wang wrote:
> On 12/13/2017 08:35 PM, Stefan Hajnoczi wrote:
> > On Wed, Dec 13, 2017 at 04:11:45PM +0800, Wei Wang wrote:
> > 
> > I think the current approach is fine for a prototype but is not suitable
> > for wider use by the community because it:
> > 1. Does not scale to multiple device types (net, scsi, blk, etc)
> > 2. Does not scale as the vhost-user protocol changes
> > 3. It is hard to make slaves run in both host userspace and the guest
> > 
> > It would be good to solve these problems so that vhost-pci can become
> > successful.  It's very hard to fix these things after the code is merged
> > because guests will depend on the device interface.
> > 
> > Here are the points in detail (in order of importance):
> > 
> > 1. Does not scale to multiple device types (net, scsi, blk, etc)
> > 
> > vhost-user is being applied to new device types beyond virtio-net.
> > There will be demand for supporting other device types besides
> > virtio-net with vhost-pci.
> > 
> > This patch series requires defining a new virtio device type for each
> > vhost-user device type.  It is a lot of work to design a new virtio
> > device.  Additionally, the new virtio device type should become part of
> > the VIRTIO standard, which can also take some time and requires writing
> > a standards document.
> > 
> > 2. Does not scale as the vhost-user protocol changes
> > 
> > When the vhost-user protocol changes it will be necessary to update the
> > vhost-pci device interface to reflect those changes.  Each protocol
> > change requires thinking how the virtio devices need to look in order to
> > support the new behavior.  Changes to the vhost-user protocol will
> > result in changes to the VIRTIO specification for the vhost-pci virtio
> > devices.
> > 
> > 3. It is hard to make slaves run in both host userspace and the guest
> > 
> > If a vhost-user slave wishes to support running in host userspace and
> > the guest then not much code can be shared between these two modes since
> > the interfaces are so different.
> > 
> > How would you solve these issues?
> 
> 1st one: I think we can factor out a common vhost-pci device layer in QEMU.
> Specific devices (net, scsi etc) emulation comes on top of it. The
> vhost-user protocol sets up VhostPCIDev only. So we will have something like
> this:
> 
> struct VhostPCINet {
>     struct VhostPCIDev vp_dev;
>     u8 mac[8];
>     ....
> }

Defining VhostPCIDev is an important step to making it easy to implement
other device types.  I'm interested is seeing how this would look either
in code or in a more detailed outline.

I wonder what the device-specific parts will be.  This patch series does
not implement a fully functional vhost-user-net device so I'm not sure.

> 2nd one: I think we need to view it the other way around: If there is a
> demand to change the protocol, then where is the demand from? I think mostly
> it is because there is some new features from the device/driver. That is, we
> first have already thought about how the virtio device looks like with the
> new feature, then we add the support to the protocol.

The vhost-user protocol will change when people using host userspace
slaves decide to change it.  They may not know or care about vhost-pci,
so the virtio changes will be an afterthought that falls on whoever
wants to support vhost-pci.

This is why I think it makes a lot more sense to stick to the vhost-user
protocol as the vhost-pci slave interface instead of inventing a new
interface on top of it.

> I'm not sure how would
> it cause not scaling well, and how using another GuestSlave-to-QemuMaster
> changes the story (we will also need to patch the GuestSlave inside the VM
> to support the vhost-user negotiation of the new feature), in comparison to
> the standard virtio feature negotiation.

Plus the VIRTIO specification needs to be updated.

And if the vhost-user protocol change affects all device types then it
may be necessary to change multiple virtio devices!  This is O(1) vs
O(N).

> 3rd one: I'm not able to solve this one, as discussed, there are too many
> differences and it's too complex. I prefer the direction of simply gating
> the vhost-user protocol and deliver to the guest what it should see (just
> what this patch series shows). You would need to solve this issue to show
> this direction is simpler :)

#3 is nice to have but not critical.

In the approach I suggested it would be done by implementing vfio
vhost-pci for libvhost-user or DPDK.

Stefan

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