Re: [virtio-comment] Re: [MASSMAIL KLMS] Re: [virtio-comment] Re: [MASSMAIL KLMS] Re: [virtio-comment] [RFC PATCH v4 2/2] virtio-vsock: SOCK_SEQPACKET description

[Stefano Garzarella <sgarzare@redhat.com>]

On Sat, Apr 03, 2021 at 12:45:54PM +0300, Arseny Krasnov wrote:
> On 31.03.2021 17:48, Stefan Hajnoczi wrote:
> > On Tue, Mar 30, 2021 at 05:24:19PM +0300, Arseny Krasnov wrote:
> > > On 30.03.2021 16:57, Stefan Hajnoczi wrote:
> > > > On Tue, Mar 30, 2021 at 12:50:06PM +0300, Arseny Krasnov wrote:
> > > > > On 30.03.2021 11:55, Stefan Hajnoczi wrote:
> > > > > > On Tue, Mar 30, 2021 at 09:15:39AM +0300, Arseny Krasnov wrote:
> > > > > > > On 30.03.2021 00:28, Stefano Garzarella wrote:
> > > > > > > > On Mon, Mar 29, 2021 at 08:33:27PM +0300, Arseny Krasnov wrote:
> > > > > > > > > On 29.03.2021 19:11, Stefan Hajnoczi wrote:
> > > > > > > > > > On Fri, Mar 26, 2021 at 12:02:50PM +0300, Arseny Krasnov wrote:
> > > > > > > > > > > @@ -98,6 +102,10 @@ \subsection{Device Operation}\label{sec:Device Types / Socket Device / Device Op
> > > > > > > > > > >  #define VIRTIO_VSOCK_OP_CREDIT_UPDATE  6
> > > > > > > > > > >  /* Request the peer to send the credit info to us */
> > > > > > > > > > >  #define VIRTIO_VSOCK_OP_CREDIT_REQUEST 7
> > > > > > > > > > > +/* Message begin for SOCK_SEQPACKET */
> > > > > > > > > > > +#define VIRTIO_VSOCK_OP_SEQ_BEGIN      8
> > > > > > > > > > > +/* Message end for SOCK_SEQPACKET */
> > > > > > > > > > > +#define VIRTIO_VSOCK_OP_SEQ_END        9
> > > > > > > > > > The struct virtio_vsock_hdr->flags field is le32 and currently unused.
> > > > > > > > > > Could 24 bits be used for a unique message id and 8 bits for flags? 1
> > > > > > > > > > flag bit could be used for end-of-message and the remaining 7 bits could
> > > > > > > > > > be reserved. That way SEQ_BEGIN and SEQ_END are not necessary.
> > > > > > > > > > Pressure
> > > > > > > > > > on the virtqueue would be reduced and performance should be comparable
> > > > > > > > > > to SOCK_STREAM.
> > > > > > > > > Well, my first versions of SOCK_SEQPACKET implementation, worked
> > > > > > > > > something like this: i used flags field of header as length of whole
> > > > > > > > > message. I discussed it with Stefano Garzarella, and he told that it
> > > > > > > > > will
> > > > > > > > > be better to use special "header" in packet's payload, to keep some
> > > > > > > > > SOCK_SEQPACKET specific data, instead of reusing packet's header
> > > > > > > > > fields.
> > > > > > > > IIRC in the first implementation SEQ_BEGIN was an empty message and we
> > > > > > > > didn't added the msg_id yet. So since we needed to carry both id and
> > > > > > > > total length, I suggested to use the payload to put these extra
> > > > > > > > information.
> > > > > > > >
> > > > > > > > IIUC what Stefan is suggesting is a bit different and I think it should
> > > > > > > > be cool to implement: we can remove the boundary packets, use only 8
> > > > > > > > bits for the flags, and add a new field to reuse the 24 unused bits,
> > > > > > > > maybe also 16 bits would be enough.
> > > > > > > > At that point we will only use the EOR flag to know the last packet.
> > > > > > > >
> > > > > > > > The main difference will be that the receiver will know the total size
> > > > > > > > only when the last packet is received.
> > > > > > > >
> > > > > > > > Do you see any issue on that approach?
> > > > > > > It will work, except we can't check that any packet of message,
> > > > > > >
> > > > > > > except last(with EOR bit) was dropped, since receiver don't know
> > > > > > >
> > > > > > > real length of message. If it is ok, then i can implement it.
> > > > > > The credit mechanism ensures that packets are not dropped, so it's not
> > > > > > necessary to check for this condition.
> > > > > >
> > > > > > By the way, is a unique message ID needed? My understanding is:
> > > > > >
> > > > > > If two messages are being sent on a socket at the same time either their
> > > > > > order is serialized (whichever message began first) or it is undefined
> > > > > > (whichever message completes first).
> > > > > If we are talking about case, when two threads writes to one socket at the same time,
> > > > >
> > > > > in Linux it is possible that two message will interleave(for vsock). But as i know, for example
> > > > >
> > > > > when TCP socket is used, both 'write()' calls will be serialized. May be it is bug of vsock: when
> > > > >
> > > > > first writer goes out of space, it will sleep. Then second writer tries to send something, but
> > > > >
> > > > > as free space is over, it will sleep too. Then, credit update is received from peer. Both sender's
> > > > >
> > > > > will be woken up, but sender which grab socket lock first will continue to send it's message.
> > > > >
> > > > > So may be we can add something like semaphore to new/vmw_vsock/af_vsock.c which will
> > > > >
> > > > > serialize two 'write()' calls: second sender enters 'write()' path, only when first left this path.
> > > > >
> > > > > My implementation doesn't care about that, because i wanted to add semaphore later, by another
> > > > >
> > > > > patch.
> > > > Yes, that is definitely an issue that the driver needs to take care of
> > > > if we don't have unique message IDs. Thanks for explaining!
> > > So may I  include patch with serializer to next version of my patchset?
> > Sounds good!
>
> There is small problem with approach, when we remove SEQ_BEGIN/SEQ_END 
>
> bounds of messages in flags field(EOR bit ) of packet header: consider case, when vhost sends data
>
> to guest(let it be 64kb). In vhost vsock driver, big buffer of packet(for example 64kb) will
>
> be splitted to small buffers, to fit guests's virtio rx descriptors(in current implementation of
>
> Linux it is 4kb). All fields of new headers in rx queue will be copied from fields of header
>
> of big packet(except len field).  Thus we get 16 headers with  EOR bit set.
> set.
>
>
> May be it is possible to:
>
> 1) Handle such bit in vhost driver(set EOR bit in flags only for last small buffer of guests rx queue)
>
> OR
>
> 2) I can remove SEQ_BEGIN, msg_len and msg_id. But keep SEQ_END op. This will be special
>
> packet which marks end of message without any payload. In this case, such packet will be
>
> processed by vhost "as is".
>
>
> What do You think?

IMHO option 1 is the best and should not be too complicated to 
implement.

Do you see a specific issue?

Thanks,
Stefano