Stefan Hajnoczi
2021-Mar-30 13:57 UTC
[virtio-comment] Re: [MASSMAIL KLMS] Re: [virtio-comment] [RFC PATCH v4 2/2] virtio-vsock: SOCK_SEQPACKET description
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! Stefan -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 488 bytes Desc: not available URL: <http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20210330/f38e54d9/attachment-0001.sig>