netdev - Re: [PATCH RFC net-next v4 6/8] virtio/vsock: support dgrams

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Date: Mon, 26 Jun 2023 17:03:15 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Bobby Eshleman <bobbyeshleman@...il.com>
Cc: Arseniy Krasnov <oxffffaa@...il.com>, 
	Bobby Eshleman <bobby.eshleman@...edance.com>, Stefan Hajnoczi <stefanha@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, "K. Y. Srinivasan" <kys@...rosoft.com>, 
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, 
	Bryan Tan <bryantan@...are.com>, Vishnu Dasa <vdasa@...are.com>, 
	VMware PV-Drivers Reviewers <pv-drivers@...are.com>, Dan Carpenter <dan.carpenter@...aro.org>, 
	Simon Horman <simon.horman@...igine.com>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org, 
	bpf@...r.kernel.org
Subject: Re: [PATCH RFC net-next v4 6/8] virtio/vsock: support dgrams

On Fri, Jun 23, 2023 at 04:37:55AM +0000, Bobby Eshleman wrote:
>On Thu, Jun 22, 2023 at 06:09:12PM +0200, Stefano Garzarella wrote:
>> On Sun, Jun 11, 2023 at 11:49:02PM +0300, Arseniy Krasnov wrote:
>> > Hello Bobby!
>> >
>> > On 10.06.2023 03:58, Bobby Eshleman wrote:
>> > > This commit adds support for datagrams over virtio/vsock.
>> > >
>> > > Message boundaries are preserved on a per-skb and per-vq entry basis.
>> >
>> > I'm a little bit confused about the following case: let vhost sends 4097 bytes
>> > datagram to the guest. Guest uses 4096 RX buffers in it's virtio queue, each
>> > buffer has attached empty skb to it. Vhost places first 4096 bytes to the first
>> > buffer of guests RX queue, and 1 last byte to the second buffer. Now IIUC guest
>> > has two skb in it rx queue, and user in guest wants to read data - does it read
>> > 4097 bytes, while guest has two skb - 4096 bytes and 1 bytes? In seqpacket there is
>> > special marker in header which shows where message ends, and how it works here?
>>
>> I think the main difference is that DGRAM is not connection-oriented, so
>> we don't have a stream and we can't split the packet into 2 (maybe we
>> could, but we have no guarantee that the second one for example will be
>> not discarded because there is no space).
>>
>> So I think it is acceptable as a restriction to keep it simple.
>>
>> My only doubt is, should we make the RX buffer size configurable,
>> instead of always using 4k?
>>
>I think that is a really good idea. What mechanism do you imagine?

Some parameter in sysfs?

>
>For sendmsg() with buflen > VQ_BUF_SIZE, I think I'd like -ENOBUFS

For the guest it should be easy since it allocates the buffers, but for
the host?

Maybe we should add a field in the configuration space that reports some
sort of MTU.

Something in addition to what Laura had proposed here:
https://markmail.org/message/ymhz7wllutdxji3e

>returned even though it is uncharacteristic of Linux sockets.
>Alternatively, silently dropping is okay... but seems needlessly
>unhelpful.

UDP takes advantage of IP fragmentation, right?
But what happens if a fragment is lost?

We should try to behave in a similar way.

>
>FYI, this patch is broken for h2g because it requeues partially sent
>skbs, so probably doesn't need much code review until we decided on the
>policy.

Got it.

Thanks,
Stefano