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

[Stefano Garzarella]

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