Virtualization - Jul 2022 - [RFC PATCH v1 0/3] virtio/vsock: use SO_RCVLOWAT to set POLLIN/POLLRDNORM

On Mon, Jul 18, 2022 at 08:12:52AM +0000, Arseniy Krasnov
wrote:
>Hello,
>
>during my experiments with zerocopy receive, i found, that in some
>cases, poll() implementation violates POSIX: when socket has non-
>default SO_RCVLOWAT(e.g. not 1), poll() will always set POLLIN and
>POLLRDNORM bits in 'revents' even number of bytes available to read
>on socket is smaller than SO_RCVLOWAT value. In this case,user sees
>POLLIN flag and then tries to read data(for example using  'read()'
>call), but read call will be blocked, because  SO_RCVLOWAT logic is
>supported in dequeue loop in af_vsock.c. But the same time,  POSIX
>requires that:
>
>"POLLIN     Data other than high-priority data may be read without
>            blocking.
> POLLRDNORM Normal data may be read without blocking."
>
>See https://www.open-std.org/jtc1/sc22/open/n4217.pdf, page 293.
>
>So, we have, that poll() syscall returns POLLIN, but read call will
>be blocked.
>
>Also in man page socket(7) i found that:
>
>"Since Linux 2.6.28, select(2), poll(2), and epoll(7) indicate a
>socket as readable only if at least SO_RCVLOWAT bytes are available."
>
>I checked TCP callback for poll()(net/ipv4/tcp.c, tcp_poll()), it
>uses SO_RCVLOWAT value to set POLLIN bit, also i've tested TCP with
>this case for TCP socket, it works as POSIX required.
I tried to look at the code and it seems that only TCP complies with it 
or am I wrong?
>
>I've added some fixes to af_vsock.c and virtio_transport_common.c,
>test is also implemented.
>
>What do You think guys?
Nice, thanks for fixing this and for the test!

I left some comments, but I think the series is fine if we will support 
it in all transports.

I'd just like to understand if it's just TCP complying with it or
I'm
missing some check included in the socket layer that we could reuse.

@David, @Jakub, @Paolo, any advice?

Thanks,
Stefano