linux-kernel - Re: [RFC PATCH v1 0/7] virtio/vsock: introduce MSG

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Message-ID: <20210804125737.kbgc6mg2v5lw25wu@steredhat>
Date:   Wed, 4 Aug 2021 14:57:37 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseny Krasnov <arseny.krasnov@...persky.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Colin Ian King <colin.king@...onical.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Norbert Slusarek <nslusarek@....net>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, oxffffaa@...il.com
Subject: Re: [RFC PATCH v1 0/7] virtio/vsock: introduce MSG_EOR flag for
 SEQPACKET

Hi Arseny,

On Mon, Jul 26, 2021 at 07:31:33PM +0300, Arseny Krasnov wrote:
>	This patchset implements support of MSG_EOR bit for SEQPACKET
>AF_VSOCK sockets over virtio transport.
>	Idea is to distinguish concepts of 'messages' and 'records'.
>Message is result of sending calls: 'write()', 'send()', 'sendmsg()'
>etc. It has fixed maximum length, and it bounds are visible using
>return from receive calls: 'read()', 'recv()', 'recvmsg()' etc.
>Current implementation based on message definition above.

Okay, so the implementation we merged is wrong right?
Should we disable the feature bit in stable kernels that contain it? Or 
maybe we can backport the fixes...

>	Record has unlimited length, it consists of multiple message,
>and bounds of record are visible via MSG_EOR flag returned from
>'recvmsg()' call. Sender passes MSG_EOR to sending system call and
>receiver will see MSG_EOR when corresponding message will be processed.
>	To support MSG_EOR new bit was added along with existing
>'VIRTIO_VSOCK_SEQ_EOR': 'VIRTIO_VSOCK_SEQ_EOM'(end-of-message) - now it
>works in the same way as 'VIRTIO_VSOCK_SEQ_EOR'. But 'VIRTIO_VSOCK_SEQ_EOR'
>is used to mark 'MSG_EOR' bit passed from userspace.

I understand that it makes sense to remap VIRTIO_VSOCK_SEQ_EOR to 
MSG_EOR to make the user understand the boundaries, but why do we need 
EOM as well?

Why do we care about the boundaries of a message within a record?
I mean, if the sender makes 3 calls:
     send(A1,0)
     send(A2,0)
     send(A3, MSG_EOR);

IIUC it should be fine if the receiver for example receives all in one 
single recv() calll with MSG_EOR set, so why do we need EOM?

Thanks,
Stefano