From: Stefano Garzarella <sgarzare@redhat.com>
Sent: Wednesday, July 20, 2022 1:23 AM
...
On Wed, Jul 20, 2022 at 05:38:03AM +0000, Arseniy Krasnov wrote:

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On 19.07.2022 15:48, Stefano Garzarella wrote:

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On Mon, Jul 18, 2022 at 08:17:31AM +0000, Arseniy Krasnov wrote:

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This callback controls setting of POLLIN,POLLRDNORM output bits
of poll() syscall,but in some cases,it is incorrectly to set it,
when socket has at least 1 bytes of available data. Use 'target'
which is already exists and equal to sk_rcvlowat in this case.

Signed-off-by: Arseniy Krasnov <redacted>
---
net/vmw_vsock/virtio_transport_common.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c

b/net/vmw_vsock/virtio_transport_common.c

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index ec2c2afbf0d0..591908740992 100644

--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c

@@ -634,7 +634,7 @@ virtio_transport_notify_poll_in(struct vsock_sock

*vsk,

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                size_t target,
                bool *data_ready_now)
{
-    if (vsock_stream_has_data(vsk))
+    if (vsock_stream_has_data(vsk) >= target)
        *data_ready_now = true;
    else
        *data_ready_now = false;

Perhaps we can take the opportunity to clean up the code in this way:

    *data_ready_now = vsock_stream_has_data(vsk) >= target;

Ack

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Anyway, I think we also need to fix the other transports (vmci and hyperv),
what do you think?

For vmci it is look clear to fix it. For hyperv i need to check it more, because it
already uses some internal target value.

Yep, I see. Maybe you can pass `target` to hvs_channel_readable() and
use it as parameter of HVS_PKT_LEN().

@Dexuan what do you think?

Thanks,
Stefano

Can we return "not supported" to set_rcvlowat for Hyper-V vsock? :-)

For Hyper-V vsock, it's easy to tell if there is at least 1 byte to read: 
please refer to hvs_channel_readable(), but it's difficult to figure out
exactly how many bytes can be read. 

In hvs_channel_readable(), hv_get_bytes_to_read() returns the total 
bytes of 0, 1 or multiple Hyper-V vsock packets: each packet has a
24-byte header (see HVS_HEADER_LEN), the payload, some padding
bytes (if the payload length is not a multiple of 8), and 8 trailing
useless bytes.

It's hard to get the total payload length because there is no API in
include/linux/hyperv.h, drivers/hv/channel.c and 
drivers/hv/ring_buffer.c that allows us to peek at the data in the
VMBus channel's ringbuffer. 

We could add such a "peek" API in drivers/hv/channel.c (see the
non-peek version of the APIs in hvs_stream_dequeue(): 
hv_pkt_iter_first() and hv_pkt_iter_next()), and examine the whole
ringbuffe to figure out the exact total payload length, but I feel it may 
not be worth the non-trivial complexity just to be POSIX-compliant --
nobody ever complained about this for the past 5 years :-) So I'm
wondering if we should allow Hyper-V vsock to not support the 
set_rcvlowat op?

Thanks,
-- Dexuan