Re: [RFC v1 0/6] virtio/vsock: introduce SOCK_DGRAM support

[Jason Wang]

在 2021/6/10 下午3:23, Stefano Garzarella 写道:
> On Thu, Jun 10, 2021 at 12:02:35PM +0800, Jason Wang wrote:
> > 在 2021/6/10 上午11:43, Jiang Wang . 写道:
> > > On Wed, Jun 9, 2021 at 6:51 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > > > 在 2021/6/10 上午7:24, Jiang Wang 写道:
> > > > > This patchset implements support of SOCK_DGRAM for virtio
> > > > > transport.
> > > > >
> > > > > Datagram sockets are connectionless and unreliable. To avoid 
> > > > > unfair contention
> > > > > with stream and other sockets, add two more virtqueues and
> > > > > a new feature bit to indicate if those two new queues exist or not.
> > > > >
> > > > > Dgram does not use the existing credit update mechanism for
> > > > > stream sockets. When sending from the guest/driver, sending packets
> > > > > synchronously, so the sender will get an error when the virtqueue 
> > > > > is full.
> > > > > When sending from the host/device, send packets asynchronously
> > > > > because the descriptor memory belongs to the corresponding QEMU
> > > > > process.
> > > >
> > > > What's the use case for the datagram vsock?
> > > One use case is for non critical info logging from the guest
> > > to the host, such as the performance data of some applications.
> >
> >
> > Anything that prevents you from using the stream socket?
> >
> >
> > > It can also be used to replace UDP communications between
> > > the guest and the host.
> >
> >
> > Any advantage for VSOCK in this case? Is it for performance (I guess 
> > not since I don't exepct vsock will be faster).
>
> I think the general advantage to using vsock are for the guest agents 
> that potentially don't need any configuration.


Right, I wonder if we really need datagram consider the host to guest 
communication is reliable.

(Note that I don't object it since vsock has already supported that, 
just wonder its use cases)


> > An obvious drawback is that it breaks the migration. Using UDP you 
> > can have a very rich features support from the kernel where vsock can't.
>
> Thanks for bringing this up!
> What features does UDP support and datagram on vsock could not support?


E.g the sendpage() and busy polling. And using UDP means qdiscs and eBPF 
can work.


> > > > > The virtio spec patch is here:
> > > > > https://www.spinics.net/lists/linux-virtualization/msg50027.html
> > > >
> > > > Have a quick glance, I suggest to split mergeable rx buffer into an
> > > > separate patch.
> > > Sure.
> > >
> > > > But I think it's time to revisit the idea of unifying the 
> > > > virtio-net and
> > > > virtio-vsock. Otherwise we're duplicating features and bugs.
> > > For mergeable rxbuf related code, I think a set of common helper
> > > functions can be used by both virtio-net and virtio-vsock. For other
> > > parts, that may not be very beneficial. I will think about more.
> > >
> > > If there is a previous email discussion about this topic, could you 
> > > send me
> > > some links? I did a quick web search but did not find any related
> > > info. Thanks.
> >
> >
> > We had a lot:
> >
> > [1] 
> > https://patchwork.kernel.org/project/kvm/patch/5BDFF537.3050806@xxxxxxxxxx/
> > [2] 
> > https://lists.linuxfoundation.org/pipermail/virtualization/2018-November/039798.html
> > [3] https://www.lkml.org/lkml/2020/1/16/2043
>
> When I tried it, the biggest problem that blocked me were all the 
> features strictly related to TCP/IP stack and ethernet devices that 
> vsock device doesn't know how to handle: TSO, GSO, checksums, MAC, 
> napi, xdp, min ethernet frame size, MTU, etc.


It depends on which level we want to share:

1) sharing codes
2) sharing devices
3) make vsock a protocol that is understood by the network core

We can start from 1), the low level tx/rx logic can be shared at both 
virtio-net and vhost-net. For 2) we probably need some work on the spec, 
probably with a new feature bit to demonstrate that it's a vsock device 
not a ethernet device. Then if it is probed as a vsock device we won't 
let packet to be delivered in the TCP/IP stack. For 3), it would be even 
harder and I'm not sure it's worth to do that.


> So in my opinion to unify them is not so simple, because vsock is not 
> really an ethernet device, but simply a socket.


We can start from sharing codes.


> But I fully agree that we shouldn't duplicate functionality and code, 
> so maybe we could find those common parts and create helpers to be 
> used by both.


Yes.

Thanks


> Thanks,
> Stefano