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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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!

Stefan

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