netdev - Re: [RFC PATCH v2 0/8] virtio/vsock: experimental zerocopy receive

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Date:   Mon, 13 Jun 2022 10:54:20 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <AVKrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>,
        Krasnov Arseniy <oxffffaa@...il.com>
Subject: Re: [RFC PATCH v2 0/8] virtio/vsock: experimental zerocopy receive

On Thu, Jun 09, 2022 at 12:33:32PM +0000, Arseniy Krasnov wrote:
>On 09.06.2022 11:54, Stefano Garzarella wrote:
>> Hi Arseniy,
>> I left some comments in the patches, and I'm adding something also here:
>Thanks for comments
>>
>> On Fri, Jun 03, 2022 at 05:27:56AM +0000, Arseniy Krasnov wrote:
>>>                              INTRODUCTION
>>>
>>>     Hello, this is experimental implementation of virtio vsock zerocopy
>>> receive. It was inspired by TCP zerocopy receive by Eric Dumazet. This API uses
>>> same idea: call 'mmap()' on socket's descriptor, then every 'getsockopt()' will
>>> fill provided vma area with pages of virtio RX buffers. After received data was
>>> processed by user, pages must be freed by 'madvise()'  call with MADV_DONTNEED
>>> flag set(if user won't call 'madvise()', next 'getsockopt()' will fail).
>>
>> If it is not too time-consuming, can we have a table/list to compare this and the TCP zerocopy?
>You mean compare API with more details?

Yes, maybe a comparison from the user's point of view to do zero-copy 
with TCP and VSOCK.

>>
>>>
>>>                                 DETAILS
>>>
>>>     Here is how mapping with mapped pages looks exactly: first page mapping
>>> contains array of trimmed virtio vsock packet headers (in contains only length
>>> of data on the corresponding page and 'flags' field):
>>>
>>>     struct virtio_vsock_usr_hdr {
>>>         uint32_t length;
>>>         uint32_t flags;
>>>         uint32_t copy_len;
>>>     };
>>>
>>> Field  'length' allows user to know exact size of payload within each sequence
>>> of pages and 'flags' allows user to handle SOCK_SEQPACKET flags(such as message
>>> bounds or record bounds). Field 'copy_len' is described below in 'v1->v2' part.
>>> All other pages are data pages from RX queue.
>>>
>>>             Page 0      Page 1      Page N
>>>
>>>     [ hdr1 .. hdrN ][ data ] .. [ data ]
>>>           |        |       ^           ^
>>>           |        |       |           |
>>>           |        *-------------------*
>>>           |                |
>>>           |                |
>>>           *----------------*
>>>
>>>     Of course, single header could represent array of pages (when packet's
>>> buffer is bigger than one page).So here is example of detailed mapping layout
>>> for some set of packages. Lets consider that we have the following sequence  of
>>> packages: 56 bytes, 4096 bytes and 8200 bytes. All pages: 0,1,2,3,4 and 5 will
>>> be inserted to user's vma(vma is large enough).
>>
>> In order to have a "userspace polling-friendly approach" and reduce number of syscall, can we allow for example the userspace to mmap at least the first header before packets arrive.
>> Then the userspace can poll a flag or other fields in the header to understand that there are new packets.
>You mean to avoid 'poll()' syscall, user will spin on some flag, provided by kernel on some mapped page? I think yes. This is ok. Also i think, that i can avoid 'madvise' call
>to clear memory mapping before each 'getsockopt()' - let 'getsockopt()' do 'madvise()' job by removing pages from previous data. In this case only one system call is needed - 'getsockopt()'.

Yes, that's right. I mean to support both, poll() for interrupt-based 
applications and the ability to actively poll a variable in the shared 
memory for applications that want to minimize latency.

Thanks,
Stefano