Re: [RFC PATCH v3 00/13] virtio/vsock: introduce SOCK_SEQPACKET support
From: Stefano Garzarella
Date: Mon Feb 01 2021 - 09:37:56 EST
On Mon, Feb 01, 2021 at 05:32:00PM +0300, Arseny Krasnov wrote:
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.
Great! Let me know if any issues we haven't considered come up.
Stefano