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

[Wang, Wei W]

On Monday, December 11, 2017 7:12 PM, Stefan Hajnoczi wrote:
> 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 <address@hidden>
> 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 
> > > >>> <address@hidden>
> > > > 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.


I think this 1:1 mapping wouldn't be easy:

1) We will have 2 Qemu side slaves to achieve this bidirectional relaying, that 
is, the working model will be 
- master to slave: Master->QemuSlave1->GuestSlave; and
- slave to master: GuestSlave->QemuSlave2->Master
QemuSlave1 and QemuSlave2 can't be the same piece of code, because QemuSlave1 
needs to do some setup with some messages, and QemuSlave2 is more likely to be 
a true "relayer" (receive and directly pass on)

2) poor re-usability of the QemuSlave and GuestSlave
We couldn’t reuse much of the QemuSlave handling code for GuestSlave.
For example, for the VHOST_USER_SET_MEM_TABLE msg, all the QemuSlave handling 
code (please see the vp_slave_set_mem_table function), won't be used by 
GuestSlave. On the other hand, GuestSlave needs an implementation to reply back 
to the QEMU device, and this implementation isn't needed by QemuSlave.
 If we want to run the same piece of the slave code in both QEMU and guest, 
then we may need "if (QemuSlave) else" in each msg handling entry to choose the 
code path for QemuSlave and GuestSlave separately.
So, ideally we wish to run (reuse) one slave implementation in both QEMU and 
guest. In practice, we will still need to handle them each case by case, which 
is no different than maintaining two separate slaves for QEMU and guest, and 
I'm afraid this would be much more complex.
 
Best,
Wei