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

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Message-ID: <ppx75eomyyb354knfkwbwin3il2ot7hf5cefwrt6ztpcbc3pps@q736cq5v4bdh>
Date:   Thu, 22 Jun 2023 18:09:12 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <oxffffaa@...il.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 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?

Thanks,
Stefano