On Fri, Feb 22, 2019 at 2:05 PM Michael S. Tsirkin <mst@redhat.com> wrote:

> On Wed, Feb 13, 2019 at 11:15:12PM -0800, Frank Yang wrote:
> > Revised the spec to clean up a straggling mention of callbacks, and
> added a
> > concrete example of how it would be used for a video codec.
> >
> > Inlined here:
>
> BTW I'd rather you started sending versioned RFC patches.
> This thread's too large already.
>
>
> Ok, I've started a new thread on virtio-comment that re-shuffles the
github links I've sent over so far, and responds to your latest questions.
Thanks Michael!


> > \subsection{Example Use Case}\label{sec:Device Types / Host Memory
> Device /
> > Example Use Case}
> >
> > Suppose the guest wants to decode a compressed video buffer.
> >
> > \begin{enumerate}
> >
> > \item Guest creates an instance for the codec vendor id / device id /
> revision.
>
> OK we'll need to see how do we come up with a way to avoid conflicts
> here, e.g. if multiple vendors will use this device.
>
> > \item Guest allocates into the PCI region via config virtqueue messages.
>
> OK so who allocates memory out of the PCI region?
> Is it the host or the guest?
> E.g. does guest say "I want X bytes" and host would respond
> "here they are, start at offset X"?
>
> > \item Guest sends a message over the ping virtqueue for the host to back
> that
> > memory.
>
> And all of these will need to be maintained on the host right?
> How many of these regions need to be supported?
>
> >
> > \item Host codec device implementation exposes codec library's buffer
> directly
> > to guest.
> >
> > \item Guest: now that the memory is host backed, the guest mmap()'s and
> >     downloads the compressed video stream directly to the host buffer.
> >
> > \item Guest: After a packet of compressed video stream is downloaded to
> the
> >     buffer, another message, like a doorbell, is sent on the ping
> virtqueue to
> >         consume existing compressed data. The ping message's offset
> field is
> >         set to the proper offset into the shared-mem object.
>
> BTW is this terminology e.g. "download", "ping message" standard somewhere?
>
> > \item Host: The ping message arrives on the host and the offset is
> resolved to
> >     a physical address and then, if possible, the physical address to a
> host
> >         pointer. Since the memory is now backed, the host pointer is also
> >         resolved.
> >
> > \item Host: Codec implementation decodes the video and puts the decoded
> frames
> >     to either a host-side display library (thus with no further guest
> >         communication necessary), or puts the raw decompressed frame to a
> >         further offset in the host buffer that the guest knows about.
> >
> > \item Guest: Continue downloading video streams and hitting the
> doorbell, or
> >     optionally, wait until the host is done first. If scheduling is not
> that
> >         big of an impact, this can be done without even any further VM
> exit, by
> >         the host writing to an agreed memory location when decoding is
> done,
> >         then the guest uses a polling sleep(N) where N is the correctly
> tuned
> >         timeout such that only a few poll spins are necessary.
> >
> >
> > \item Guest: Or, the host can send back on the event virtqueue
> \field{revents}
> >     and the guest can perform a blocking read() for it.
> >
> > \end{enumerate}
> >
> > The unique / interesting aspects of virtio-hostmem are demonstrated:
> >
> > \begin{enumerate}
> >
> > \item During instance creation the host was allowed to reject the
> request if
> >     the codec device did not exist on host.
> >
> > \item The host can expose a codec library buffer directly to the guest,
> > allowing the guest to write into it with zero copy and the host to
> decompress
> > again without copying.
> >
> > \item Large bidirectional transfers are possible with zero copy.
>
> However just to make sure, sending small amounts of data
> is slower since you get to do all the mmap dance.
>
>
> > \item Large bidirectional transfers are possible without scatterlists,
> because
> >     the memory is always physically contiguous.
>
> It might get fragmented though. I think it would be up to
> host to try and make sure it's not too fragmented, right?
>
>
> > \item It is not necessary to use socket datagrams or data streams to
> >     communicate the ping messages; they can be raw structs fresh off the
> >         virtqueue.
>
> OK and ping messages are all fixed size?
>
>
> > \item After decoding, the guest has the option but not the requirement
> to wait
> >     for the host round trip, allowing for async operation of the codec.
> >
> > \item The guest has the option but not the requirement to wait for the
> host
> >     round trip, allowing for async operation of the codec.
> >
> > \end{enumerate}
>
> OK I still owe you that write-up about vhost pci.  will try to complete
> that early next week. But generally if I got it right that the host
> allocates buffers then what you describe does seem to fit a bit better
> with the vhost pci host/guest interface idea.
>
> One question that was asked about vhost pci is whether it is in fact
> necessary to share a device between multiple applications.
> Or is it enough to just have one id per device?
>
> Thanks!
> --
> MST
>