Re: [PATCH v7 12/21] multi-process: Connect Proxy Object with device in the remote process

[Jag Raman]

> On Jul 27, 2020, at 9:18 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> On Fri, Jul 24, 2020 at 12:57:22PM -0400, Jag Raman wrote:
>>> On Jul 1, 2020, at 5:20 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>> On Sat, Jun 27, 2020 at 10:09:34AM -0700, elena.ufimtseva@oracle.com wrote:
>>> In a 1:1 model the CONNECT_DEV message is not necessary because each
>>> socket is already associated with a specific remote device (e.g. qemu -M
>>> remote -object mplink,dev=lsi-scsi-1,sockpath=/tmp/lsi-scsi-1.sock).
>>> Connecting to the socket already indicates which device we are talking
>>> to.
>>> 
>>> The N:1 model will work but it's more complex. There is a main socket
>>> that is used for CONNECT_DEV (anything else?) and we need to worry about
>>> the lifecycle of the per-device sockets that are passed over the main
>>> socket.
>> 
>> The main socket is only used for CONNECT_DEV. The CONNECT_DEV
>> message sticks a fd to the remote device.
>> 
>> We are using the following command-line in the remote process:
>> qemu-system-x86_64 -machine remote,fd=4 -device lsi53c895a,id=lsi1 ...
>> 
>> The alternative approach would be to let the orchestrator to assign fds for
>> each remote device. In this approach, we would specify an ‘fd’ for each
>> device object like below:
>> qemu-system-x86_64 -machine remote -device lsi53c895a,id=lsi1,fd=4 …
>> 
>> The alternative approach would entail changes to the DeviceState struct
>> and qdev_device_add() function, which we thought is not preferable. Could
>> you please share your thoughts on this?
> 
> I suggest dropping multi-device support for now. It will be implemented
> differently with VFIO-over-socket anyway, so it's not worth investing
> much time into.
> 
> The main socket approach needs authentication support if multiple guests
> share a remote device emulation process. Otherwise guest A can access
> guest B's devices.
> 
> It's simpler if each device has a separate UNIX domain socket. It is not
> necessary to modify lsi53c895a in order to do this. Either the socket
> can be associated with the remote PCIe port (although I think the
> current code implements the older PCI Local Bus instead of PCIe) or a
> separate -object mpqlink,device=lsi1,fd=4 object can be defined (I think
> that's the syntax I've shared in the past).
> 
> For now though, just using the -machine remote,fd=4 approach is fine -
> but limited to 1 device.

Hi Stefan,

Thanks for your response.

We didn’t quite get the purpose of the “object” when you proposed the
command-line. But we understand it now.

We also agree to wait for the authentication protocol before enabling
multiple devices in the same remote process. Therefore we have limited
the number of devices to 1.

Even with one device, we still had the problem of attaching the file
descriptor with a device int the remote end. The object idea you proposed
seemed to fit well with connecting the fd with device. We have added this
object in v8, which we just sent out.

We have dropped the main-socket/control-channel as it’s not needed
anymore.

We look forward to your responses to v8.

Thank you very much!

> 
> Stefan