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

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Message-ID: <9f0b6bba-701c-a95d-d326-bb207e319f2a@gmail.com>
Date:   Thu, 22 Jun 2023 21:46:26 +0300
From:   Arseniy Krasnov <oxffffaa@...il.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     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 22.06.2023 19:09, 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.

Ah, I see, idea is that any "corruptions" of data could be considered as
"DGRAM is not reliable anyway, so that's it" :)

> 
> My only doubt is, should we make the RX buffer size configurable,
> instead of always using 4k?

I guess this is useful only for DGRAM usage, when we want to tune buffers
for some specific case - may be for exact length of messages (for example if we have
4096 buffers, while senders wants to send 5000 bytes always by each 'send()' - I think it
will be really strange that reader ALWAYS dequeues 4096 and 4 bytes as two packets).
For stream types of socket I think size of rx buffers is not big deal in most of cases.

Thanks, Arseniy

> 
> Thanks,
> Stefano
>