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

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Date:   Thu, 29 Jun 2023 14:30:35 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Bobby Eshleman <bobbyeshleman@...il.com>
Cc:     linux-hyperv@...r.kernel.org,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
        VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
        Simon Horman <simon.horman@...igine.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Eric Dumazet <edumazet@...gle.com>,
        Dan Carpenter <dan.carpenter@...aro.org>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Bryan Tan <bryantan@...are.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Arseniy Krasnov <oxffffaa@...il.com>,
        Vishnu Dasa <vdasa@...are.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH RFC net-next v4 6/8] virtio/vsock: support dgrams

On Tue, Jun 27, 2023 at 01:19:43AM +0000, Bobby Eshleman wrote:
>On Mon, Jun 26, 2023 at 05:03:15PM +0200, Stefano Garzarella wrote:
>> 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?
>>
>
>I comment more on this below.
>
>> >
>> > 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
>>
>
>That sounds good to me.
>
>IIUC vhost exposes the limit via the configuration space, and the guest
>can configure the RX buffer size up to that limit via sysfs?
>
>> > 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.
>>
>
>AFAICT in UDP the sending socket will see EHOSTUNREACH on its error
>queue and the packet will be dropped.
>
>For more details:
>- the IP defragmenter will emit an ICMP_TIME_EXCEEDED from ip_expire()
>  if the fragment queue is not completed within time.
>- Upon seeing ICMP_TIME_EXCEEDED, the sending stack will then add
>  EHOSTUNREACH to the socket's error queue, as seen in __udp4_lib_err().
>
>Given some updated man pages I think enqueuing EHOSTUNREACH is okay for
>vsock too. This also reserves ENOBUFS/ENOMEM only for shortage on local
>buffers / mem.
>
>What do you think?

Yep, makes sense to me!

Stefano