Re: [RFC PATCH v1 2/3] virtio/vsock: use 'target' in notify_poll_in, callback.

[Arseniy Krasnov]

On 20.07.2022 21:54, Dexuan Cui wrote:
>> From: Stefano Garzarella <sgarzare@xxxxxxxxxx>
>> Sent: Wednesday, July 20, 2022 1:23 AM
>> ...
>> On Wed, Jul 20, 2022 at 05:38:03AM +0000, Arseniy Krasnov wrote:
>>> On 19.07.2022 15:48, Stefano Garzarella wrote:
>>>> On Mon, Jul 18, 2022 at 08:17:31AM +0000, Arseniy Krasnov wrote:
>>>>> 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 <AVKrasnov@xxxxxxxxxxxxxx>
>>>>> ---
>>>>> 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
>>>>> 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,
>>>>>                 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
>>>>
>>>> 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?
I see, i can prepare patch for this in this patchset in v2. Call to set SO_RCVLOWAT for Hyper-V
transport will return EOPNOTSUPP.

Thank You
> 
> Thanks,
> -- Dexuan
>