Re: [RFC PATCH v3 00/13] virtio/vsock: introduce SOCK_SEQPACKET support

[Arseny Krasnov]

On 01.02.2021 17:23, Stefano Garzarella wrote:
> On Mon, Feb 01, 2021 at 04:57:18PM +0300, Arseny Krasnov wrote:
>> On 01.02.2021 14:02, Stefano Garzarella wrote:
>>> On Fri, Jan 29, 2021 at 06:52:23PM +0300, Arseny Krasnov wrote:
>>>> On 29.01.2021 12:26, Stefano Garzarella wrote:
>>>>> On Fri, Jan 29, 2021 at 09:41:50AM +0300, Arseny Krasnov wrote:
>>>>>> On 28.01.2021 20:19, Stefano Garzarella wrote:
>>>>>>> Hi Arseny,
>>>>>>> I reviewed a part, tomorrow I hope to finish the other patches.
>>>>>>>
>>>>>>> Just a couple of comments in the TODOs below.
>>>>>>>
>>>>>>> On Mon, Jan 25, 2021 at 02:09:00PM +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, new packet operation was added: it marks start of record (with
>>>>>>>> record length in header), such packet doesn't carry any data.  To send
>>>>>>>> record, packet with start marker is sent first, then all data is sent
>>>>>>>> as usual 'RW' packets. On receiver's side, length of record is known
>>>>>>> >from packet with start record marker. Now as  packets of one socket
>>>>>>>> are not reordered neither on vsock nor on vhost transport layers, such
>>>>>>>> marker 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.
>>>>>>>>
>>>>>>>> Arseny Krasnov (13):
>>>>>>>>  af_vsock: prepare for SOCK_SEQPACKET support
>>>>>>>>  af_vsock: prepare 'vsock_connectible_recvmsg()'
>>>>>>>>  af_vsock: implement SEQPACKET rx loop
>>>>>>>>  af_vsock: implement send logic for SOCK_SEQPACKET
>>>>>>>>  af_vsock: rest of SEQPACKET support
>>>>>>>>  af_vsock: update comments for stream sockets
>>>>>>>>  virtio/vsock: dequeue callback for SOCK_SEQPACKET
>>>>>>>>  virtio/vsock: fetch length for SEQPACKET record
>>>>>>>>  virtio/vsock: add SEQPACKET receive logic
>>>>>>>>  virtio/vsock: rest of SOCK_SEQPACKET support
>>>>>>>>  virtio/vsock: setup SEQPACKET ops for transport
>>>>>>>>  vhost/vsock: setup SEQPACKET ops for transport
>>>>>>>>  vsock_test: add SOCK_SEQPACKET tests
>>>>>>>>
>>>>>>>> drivers/vhost/vsock.c                   |   7 +-
>>>>>>>> include/linux/virtio_vsock.h            |  12 +
>>>>>>>> include/net/af_vsock.h                  |   6 +
>>>>>>>> include/uapi/linux/virtio_vsock.h       |   9 +
>>>>>>>> net/vmw_vsock/af_vsock.c                | 543 ++++++++++++++++------
>>>>>>>> net/vmw_vsock/virtio_transport.c        |   4 +
>>>>>>>> net/vmw_vsock/virtio_transport_common.c | 295 ++++++++++--
>>>>>>>> tools/testing/vsock/util.c              |  32 +-
>>>>>>>> tools/testing/vsock/util.h              |   3 +
>>>>>>>> tools/testing/vsock/vsock_test.c        | 126 +++++
>>>>>>>> 10 files changed, 862 insertions(+), 175 deletions(-)
>>>>>>>>
>>>>>>>> TODO:
>>>>>>>> - Support for record integrity control. As transport could drop some
>>>>>>>>   packets, something like "record-id" and record end marker need to
>>>>>>>>   be implemented. Idea is that SEQ_BEGIN packet carries both record
>>>>>>>>   length and record id, end marker(let it be SEQ_END) carries only
>>>>>>>>   record id. To be sure that no one packet was lost, receiver checks
>>>>>>>>   length of data between SEQ_BEGIN and SEQ_END(it must be same with
>>>>>>>>   value in SEQ_BEGIN) and record ids of SEQ_BEGIN and SEQ_END(this
>>>>>>>>   means that both markers were not dropped. I think that easiest way
>>>>>>>>   to implement record id for SEQ_BEGIN is to reuse another field of
>>>>>>>>   packet header(SEQ_BEGIN already uses 'flags' as record length).For
>>>>>>>>   SEQ_END record id could be stored in 'flags'.
>>>>>>> I don't really like the idea of reusing the 'flags' field for this
>>>>>>> purpose.
>>>>>>>
>>>>>>>>     Another way to implement it, is to move metadata of both SEQ_END
>>>>>>>>   and SEQ_BEGIN to payload. But this approach has problem, because
>>>>>>>>   if we move something to payload, such payload is accounted by
>>>>>>>>   credit logic, which fragments payload, while payload with record
>>>>>>>>   length and id couldn't be fragmented. One way to overcome it is to
>>>>>>>>   ignore credit update for SEQ_BEGIN/SEQ_END packet.Another solution
>>>>>>>>   is to update 'stream_has_space()' function: current implementation
>>>>>>>>   return non-zero when at least 1 byte is allowed to use,but updated
>>>>>>>>   version will have extra argument, which is needed length. For 'RW'
>>>>>>>>   packet this argument is 1, for SEQ_BEGIN it is sizeof(record len +
>>>>>>>>   record id) and for SEQ_END it is sizeof(record id).
>>>>>>> Is the payload accounted by credit logic also if hdr.op is not
>>>>>>> VIRTIO_VSOCK_OP_RW?
>>>>>> Yes, on send any packet with payload could be fragmented if
>>>>>>
>>>>>> there is not enough space at receiver. On receive 'fwd_cnt' and
>>>>>>
>>>>>> 'buf_alloc' are updated with header of every packet. Of course,
>>>>>>
>>>>>> to every such case i've described i can add check for 'RW'
>>>>>>
>>>>>> packet, to exclude payload from credit accounting, but this is
>>>>>>
>>>>>> bunch of dumb checks.
>>>>>>
>>>>>>> I think that we can define a specific header to put after the
>>>>>>> virtio_vsock_hdr when hdr.op is SEQ_BEGIN or SEQ_END, and in this header
>>>>>>> we can store the id and the length of the message.
>>>>>> I think it is better than use payload and touch credit logic
>>>>>>
>>>>> Cool, so let's try this option, hoping there aren't a lot of issues.
>>>> If i understand, current implementation has 'struct 
>>>> virtio_vsock_hdr',
>>>>
>>>> then i'll add 'struct virtio_vsock_hdr_seq' with message length and id.
>>>>
>>>> After that, in 'struct virtio_vsock_pkt' which describes packet, field for
>>>>
>>>> header(which is 'struct virtio_vsock_hdr') must be replaced with new
>>>>
>>>> structure which  contains both 'struct virtio_vsock_hdr' and 'struct
>>>>
>>>> virtio_vsock_hdr_seq', because header field of 'struct virtio_vsock_pkt'
>>>>
>>>> is buffer for virtio layer. After it all accesses to header(for example to
>>>>
>>>> 'buf_alloc' field will go accross new  structure with both headers:
>>>>
>>>> pkt->hdr.buf_alloc   ->   pkt->extended_hdr.classic_hdr.buf_alloc
>>>>
>>>> May be to avoid this, packet's header could be allocated dynamically
>>>>
>>>> in the same manner as packet's buffer? Size of allocation is always
>>>>
>>>> sizeof(classic header) + sizeof(seq header). In 'struct virtio_vsock_pkt'
>>>>
>>>> such header will be implemented as union of two pointers: class header
>>>>
>>>> and extended header containing classic and seq header. Which pointer
>>>>
>>>> to use is depends on packet's op.
>>> I think that the 'classic header' can stay as is, and the extended
>>> header can be dynamically allocated, as we do for the payload.
>>>
>>> But we have to be careful what happens if the other peer doesn't support
>>> SEQPACKET and if it counts this extra header as a payload for the credit
>>> mechanism.
>> You mean put extra header to payload(buffer of second virtio desc),
>>
>> in this way on send/receive auxiliary 'if's are needed to avoid credit
>>
>> logic(or set length field in header of such packets to 0). But what
>>
>> about placing extra header after classic header in buffer of first virtio
>>
>> desc? In this case extra header is not payload and credit works as is.
>>
>> Or it is critical, that size of first buffer will be not same as size of
>>
>> classic header?
> We need to think about compatibility with old drivers.
Yes, compatibility seems to be a trouble.
>
> What would happen in this case?
>
> I think it's easier to use the second buffer, usually used for the 
> payload, to carry the extra header. Also, we can leave hdr.len = 0, so 
> we are sure that it is not counted in credit mechanism.

Ok, that one of possible solutions. I just wanted to inform you,

that way i'll use in v4

> If the driver supports SEQPACKET, it knows it must fetch extra header 
> when it must handle SEQ_BEGIN/SEQ_END.
>
> If it is not clear, I'll try to provide a simple PoC of a patch.

No, it is clear for me, i'll implement it in v4 also take care of

review comments.

Thank You

>
> Thanks,
> Stefano
>
>