Re: [External] Re: vsock virtio: questions about supporting DGRAM type

["Jiang Wang ." <jiang.wang@bytedance.com>]

Hi Michael,

Sorry for the late reply. I just saw your email yesterday somehow.

I read the email thread you mentioned, and I think the issue with
dgram is that it may drop packets because the sender cannot track the
tx_cnt with subtracting it from peer_fwd_cnt.

I agree with Stefan that the dgram is a best-effort service and may
drop packets. For the sender, I just add a maximum buffer size to
limit the memory usage. On the receiving side, I reuse the existing
virtio_transport_inc_rx_pkt() that Stefano added a year ago to limit
the memory usage. This will avoid denial of service attack to the
other end (host or guest VM).

For the application of dgram, we will use it for some remote logging
application. The application running in the VM will write some logs to
a server running on the host. This is  one way communication and the
log is not critical.

Regards,

Jiang

On Tue, Feb 23, 2021 at 1:53 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Feb 11, 2021 at 10:04:34PM -0800, Jiang Wang . wrote:
> > Hi guys,
> >
> > I am working on supporting DGRAM type for virtio/vhost vsock. I
> > already did some work and a draft code is here (which passed my tests,
> > but still need some cleanup and only works from host to guest as of
> > now, will add host to guest soon):
> > https://github.com/Jiang1155/linux/commit/4e89736e0bce15496460ff411cb4694b143d1c3d
> > qemu changes are here:
> > https://github.com/Jiang1155/qemu/commit/7ab778801e3e8969ab98e44539943810a2fb03eb
> >
> > Today, I just noticed that the Asias had an old version of virtio
> > which had both dgram and stream support, see this link:
> > https://kvm.vger.kernel.narkive.com/BMvH9eEr/rfc-v2-0-7-introduce-vm-sockets-virtio-transport#post1
> >
> > But somehow, the dgram part seems never merged to upstream linux (the
> > stream part is merged). If so, does anyone know what is the reason for
> > this? Did we drop dgram support for some specific reason or the code
> > needs some improvement?
>
> It's not just code it's the spec that needs work.
>
> See some issues pointed out here:
>
> https://lists.oasis-open.org/archives/virtio-dev/201506/msg00003.html
>
>
>
> > My current code differs from Asias' code in some ways. It does not use
> > credit and does not support fragmentation.  It basically adds two virt
> > queues and re-uses the existing functions for tx and rx ( there is
> > somewhat duplicate code for now, but I will try to make common
> > functions to reduce it). If we still want to support dgram in upstream
> > linux, which way do you guys recommend? If necessary, I can try to
> > base on Asias' old code and continue working on it. If there is
> > anything unclear, just let me know. Thanks.
> >
> > Regards,
> >
> > Jiang
>
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