Re: [Qemu-devel] [virtio-comment] Re: [PATCH] *** Vhost-pci RFC v2 ***

[Wei Wang <wei.w.wang@intel.com>]

On 09/01/2016 09:05 PM, Marc-André Lureau wrote:
> On Thu, Sep 1, 2016 at 4:13 PM Wei Wang <wei.w.wang@intel.com 
> <mailto:wei.w.wang@intel.com>> wrote:
>
>     On 09/01/2016 04:49 PM, Marc-André Lureau wrote:
>     > Hi
>     >
>     > On Thu, Sep 1, 2016 at 12:19 PM Wei Wang <wei.w.wang@intel.com
>     <mailto:wei.w.wang@intel.com>
>     > <mailto:wei.w.wang@intel.com <mailto:wei.w.wang@intel.com>>> wrote:
>     >
>     >     On 08/31/2016 08:30 PM, Marc-André Lureau wrote:
>     >
>     >>     - If it could be made not pci-specific, a better name for the
>     >>     device could be simply "driver": the driver of a virtio device.
>     >>     Or the "slave" in vhost-user terminology - consumer of virtq. I
>     >>     think you prefer to call it "backend" in general, but I find it
>     >>     more confusing.
>     >
>     >     Not really. A virtio device has it own driver (e.g. a virtio-net
>     >     driver for a virtio-net device). A vhost-pci device plays
>     the role
>     >     of a backend (just like vhost_net, vhost_user) for a virtio
>     >     device. If we use the "device/driver" naming convention, the
>     >     vhost-pci device is part of the "device". But I actually
>     prefer to
>     >     use "frontend/backend" :) If we check the QEMU's
>     >     doc/specs/vhost-user.txt, it also uses "backend" to describe.
>     >
>     >
>     > Yes, but it uses "backend" freely without any definition and to name
>     > eventually different things. (at least "slave" is being defined
>     as the
>     > consumer of virtq, but I think some people don't like to use
>     that word).
>     >
>
>     I think most people know the concept of backend/frontend, that's
>     probably the reason why they usually don't explicitly explain it in a
>     doc. If you guys don't have an objection, I suggest to use it in the
>     discussion :)  The goal here is to get the design finalized first.
>     When
>     it comes to the final spec wording phase, we can decide which
>     description is more proper.
>
>
> "backend" is too broad for me. Instead I would stick to something 
> closer to what we want to name and define. If it's the consumer of 
> virtq, then why not call it that way.

     OK. Let me get used to it (provider VM - frontend, consumer VM - 
backend).

>
>     > Have you thought about making the device not pci specific? I don't
>     > know much about mmio devices nor s/390, but if devices can hotplug
>     > their own memory (I believe mmio can), then it should be possible to
>     > define a device generic enough.
>
>     Not yet. I think the main difference would be the way to map the
>     frontend VM's memory (in our case, we use a BAR). Other things
>     should be
>     generic.
>
>
> I hope some more knowledgeable people will chime in.

That would be great.

>
>
>     >
>     >>     - Why is it required or beneficial to support multiple
>     "frontend"
>     >>     devices over the same "vhost-pci" device? It could simplify
>     >>     things if it was a single device. If necessary, that could also
>     >>     be interesting as a vhost-user extension.
>     >
>     >     We call it "multiple backend functionalities" (e.g.
>     vhost-pci-net,
>     >     vhost-pci-scsi..). A vhost-pci driver contains multiple such
>     >     backend functionalities, because in this way they can reuse
>     >     (share) the same memory mapping. To be more precise, a vhost-pci
>     >     device supplies the memory of a frontend VM, and all the backend
>     >     functionalities need to access the same frontend VM memory,
>     so we
>     >     consolidate them into one vhost-pci driver to use one vhost-pci
>     >     device.
>     >
>     >
>     > That's what I imagined. Do you have a use case for that?
>
>     Currently, we only have the network use cases. I think we can
>     design it
>     that way (multple backend functionalities), which is more generic (not
>     just limited to network usages). When implementing it, we can
>     first have
>     the network backend functionality (i.e. vhost-pci-net) implemented. In
>     the future, if people are interested in other backend
>     functionalities, I
>     think it should be easy to add them.
>
>
> My question is not about the support of various kind of devices (that 
> is clearly a worthy goal to me) but to have support simultaneously of 
> several frontend/provider devices on the same vhost-pci device: is 
> this required or necessary? I think it would simplify things if it was 
> 1-1 instead, I would like to understand why you propose a different 
> design.

It is not required, but necessary, I think. As mentioned above, those 
consumer-side functionalities basically access the same provider VM's 
memory, so I think one vhost-pci device is enough to hold that memory. 
When it comes to the consumer guest kernel, we only need to ioremap that 
memory once. Also, a pair of controlq-s is enough to handle the control 
path messages between all those functionalities and the QEMU. I think 
the design also looks compact in this way. what do you think?

If we make it an N-N model (each functionality has a vhost-pci device), 
then the QEMU and guest kernel need to repeat those memory setup things 
N times.

Best,
Wei