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

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:

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On Wed, Jun 9, 2021 at 6:51 PM Jason Wang [off-list ref] wrote:

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在 2021/6/10 上午7:24, Jiang Wang 写道:

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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?

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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.

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?

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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.

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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@huawei.com/
[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.

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

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.

Thanks,
Stefano

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