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SubjectRe: [RFC PATCH v2 0/9] vsock: updates for SO_RCVLOWAT handling
On 02.08.2022 08:31, Vishnu Dasa wrote:
>> On Jul 27, 2022, at 11:08 PM, Arseniy Krasnov <> wrote:
>> On 27.07.2022 15:37, Stefano Garzarella wrote:
>>> Hi Arseniy,
>>> On Mon, Jul 25, 2022 at 07:54:05AM +0000, Arseniy Krasnov wrote:
>>>> Hello,
>>>> This patchset includes some updates for SO_RCVLOWAT:
>>>> 1) af_vsock:
>>>> 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;;sdata=NrbycCcVXV9Tz8NRDYBpnDx7KpFF6BZpSRbuhz1IfJ4%3D&amp;reserved=0, 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've added some fixes to af_vsock.c and virtio_transport_common.c,
>>>> test is also implemented.
>>>> 2) virtio/vsock:
>>>> It adds some optimization to wake ups, when new data arrived. Now,
>>>> SO_RCVLOWAT is considered before wake up sleepers who wait new data.
>>>> There is no sense, to kick waiter, when number of available bytes
>>>> in socket's queue < SO_RCVLOWAT, because if we wake up reader in
>>>> this case, it will wait for SO_RCVLOWAT data anyway during dequeue,
>>>> or in poll() case, POLLIN/POLLRDNORM bits won't be set, so such
>>>> exit from poll() will be "spurious". This logic is also used in TCP
>>>> sockets.
>>> Nice, it looks good!
>> Thank You!
>>>> 3) vmci/vsock:
>>>> Same as 2), but i'm not sure about this changes. Will be very good,
>>>> to get comments from someone who knows this code.
>>> I CCed VMCI maintainers to the patch and also to this cover, maybe better to keep them in the loop for next versions.
>>> (Jorgen's and Rajesh's emails bounced back, so I'm CCing here only Bryan, Vishnu, and
>> Ok, i'll CC them in the next version
>>>> 4) Hyper-V:
>>>> As Dexuan Cui mentioned, for Hyper-V transport it is difficult to
>>>> support SO_RCVLOWAT, so he suggested to disable this feature for
>>>> Hyper-V.
>>> I left a couple of comments in some patches, but it seems to me to be in a good state :-)
>>> I would just suggest a bit of a re-organization of the series (the patches are fine, just the order):
>>> - introduce vsock_set_rcvlowat()
>>> - disabling it for hv_sock
>>> - use 'target' in virtio transports
>>> - use 'target' in vmci transports
>>> - use sock_rcvlowat in vsock_poll()
>>> I think is better to pass sock_rcvlowat() as 'target' when the
>>> transports are already able to use it
>>> - add vsock_data_ready()
>>> - use vsock_data_ready() in virtio transports
>>> - use vsock_data_ready() in vmci transports
>>> - tests
>>> What do you think?
>> No problem! I think i can wait for reply from VMWare guys before preparing v3
> Looks fine to me, especially the VMCI parts. Please send v3, and we can test it
> from VMCI point of view as well.
Great, thank you for reply. I'll prepare v3 ASAP and You will be CCed

> Thanks,
> Vishnu

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