Stefano Garzarella
2021-Feb-24 08:23 UTC
[RFC PATCH v5 00/19] virtio/vsock: introduce SOCK_SEQPACKET support
On Wed, Feb 24, 2021 at 07:29:25AM +0300, Arseny Krasnov wrote:> >On 23.02.2021 17:50, Stefano Garzarella wrote: >> On Mon, Feb 22, 2021 at 03:23:11PM +0100, Stefano Garzarella wrote: >>> Hi Arseny, >>> >>> On Thu, Feb 18, 2021 at 08:33:44AM +0300, Arseny Krasnov wrote: >>>> This patchset impelements support of SOCK_SEQPACKET for virtio >>>> transport. >>>> As SOCK_SEQPACKET guarantees to save record boundaries, so to >>>> do it, two new packet operations were added: first for start of record >>>> and second to mark end of record(SEQ_BEGIN and SEQ_END later). Also, >>>> both operations carries metadata - to maintain boundaries and payload >>>> integrity. Metadata is introduced by adding special header with two >>>> fields - message count and message length: >>>> >>>> struct virtio_vsock_seq_hdr { >>>> __le32 msg_cnt; >>>> __le32 msg_len; >>>> } __attribute__((packed)); >>>> >>>> This header is transmitted as payload of SEQ_BEGIN and SEQ_END >>>> packets(buffer of second virtio descriptor in chain) in the same way as >>>> data transmitted in RW packets. Payload was chosen as buffer for this >>>> header to avoid touching first virtio buffer which carries header of >>>> packet, because someone could check that size of this buffer is equal >>>> to size of packet header. To send record, packet with start marker is >>>> sent first(it's header contains length of record and counter), then >>>> counter is incremented and all data is sent as usual 'RW' packets and >>>> finally SEQ_END is sent(it also carries counter of message, which is >>>> counter of SEQ_BEGIN + 1), also after sedning SEQ_END counter is >>>> incremented again. On receiver's side, length of record is known from >>>> packet with start record marker. To check that no packets were dropped >>>> by transport, counters of two sequential SEQ_BEGIN and SEQ_END are >>>> checked(counter of SEQ_END must be bigger that counter of SEQ_BEGIN by >>>> 1) and length of data between two markers is compared to length in >>>> SEQ_BEGIN header. >>>> Now as packets of one socket are not reordered neither on >>>> vsock nor on vhost transport layers, such markers allows to restore >>>> original record on receiver's side. If user's buffer is smaller that >>>> record length, when all out of size data is dropped. >>>> Maximum length of datagram is not limited as in stream socket, >>>> because same credit logic is used. Difference with stream socket is >>>> that user is not woken up until whole record is received or error >>>> occurred. Implementation also supports 'MSG_EOR' and 'MSG_TRUNC' flags. >>>> Tests also implemented. >>> I reviewed the first part (af_vsock.c changes), tomorrow I'll review >>> the rest. That part looks great to me, only found a few minor issues. >> I revieiwed the rest of it as well, left a few minor comments, but I >> think we're well on track. >> >> I'll take a better look at the specification patch tomorrow. >Great, Thank You >> >> Thanks, >> Stefano >> >>> In the meantime, however, I'm getting a doubt, especially with regard >>> to other transports besides virtio. >>> >>> Should we hide the begin/end marker sending in the transport? >>> >>> I mean, should the transport just provide a seqpacket_enqueue() >>> callbacl? >>> Inside it then the transport will send the markers. This is because >>> some transports might not need to send markers. >>> >>> But thinking about it more, they could actually implement stubs for >>> that calls, if they don't need to send markers. >>> >>> So I think for now it's fine since it allows us to reuse a lot of >>> code, unless someone has some objection. > >I thought about that, I'll try to implement it in next version. Let's see...If you want to discuss it first, write down the idea you want to implement, I wouldn't want to make you do unnecessary work. :-) Cheers, Stefano