Re: [virtio-dev] [PATCH v3 0/4] introduce virtio vhost-user backend device type

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 05, 2019 at 01:32:14AM +0300, Nikos Dragazis wrote:
> On 31/7/19 11:19 μ.μ., Michael S. Tsirkin wrote:
> > On Wed, Jul 31, 2019 at 05:49:57PM +0300, Nikos Dragazis wrote:
> >> On 21/6/19 11:43 μ.μ., Michael S. Tsirkin wrote:
> >>> On Wed, Jun 19, 2019 at 03:54:30PM +0100, Stefan Hajnoczi wrote:
> >>>> On Sat, May 11, 2019 at 09:47:36PM +0300, Nikos Dragazis wrote:
> >>>>> Hi everyone,
> >>>>>
> >>>>> this PATCH presents an updated version of the RFC virtio device spec for
> >>>>> the virtio-vhost-user device. The initial RFC implementation can be
> >>>>> found here: [1].
> >>>>>
> >>>>> This PATCH is split into four parts:
> >>>>>
> >>>>> 1. the first commit is just a refactored version of the initial RFC
> >>>>> implementation [1]. I just moved the device spec into a separate .tex
> >>>>> file and changed the device id from #24 to #28 since the ids #24-#27 are
> >>>>> reserved.
> >>>>>
> >>>>> 2. the second commit fixes some minor issues with the device spec. This
> >>>>> is quite straightforward.
> >>>>>
> >>>>> 3. the third commit enhances the notification capability with some
> >>>>> device/driver requirements. This makes sense because the notification
> >>>>> capability behaves similarly to the MSI-X capability, thereby having
> >>>>> some requirements that need to be mentioned.
> >>>>>
> >>>>> 4. the fourth commit synchronizes the shared memory capability with a
> >>>>> recent patch [2] that attempts to standardize a standalone
> >>>>> SHARED_MEMORY_CFG virtio capability.
> >>>>>
> >>>>> v3 changes:
> >>>>>  * Device Requirements for the Notification Capability: point out the
> >>>>>    difference between the MSI-X Table Size stored in the Message Control
> >>>>>    register of the MSI-X capability structure and the actual MSI-X Table
> >>>>>    Size
> >>>>>
> >>>>> v2 changes:
> >>>>>  * Change device id from #25 to #28
> >>>>>
> >>>>> Looking forward to your comments.
> >>>> I'm happy with v3.
> >>>>
> >>>> Any other comments or shall we proceed to a vote?
> >>>>
> >>>> Stefan
> >>> I think we need to wait for shared memory part to be finalized, right?
> >>>
> >> Michael,
> >>
> >> given that David's patchset for the shared memory regions has been
> >> approved, I think we can start discussing on the spec for the
> >> virtio-vhost-user device. A link to the latest version is here: [1].
> >>
> >> I really think that this device is useful and should be part of the
> >> VIRTIO specification. I will submit a fifth version of the patchset soon
> >> with some minor changes that seem reasonable to me.
> >>
> >> Best regards,
> >> Nikos
> >>
> >> [1] https://lists.oasis-open.org/archives/virtio-dev/201906/msg00036.html
> > One part I dislike there is "Additional Device Resources over PCI".
> > That mostly seems to deal with allocating interrupts.
> 
> Actually, it deals with allocating notification addresses (doorbells)
> and device interrupts for the vhost-user virtqueues. The
> virtio-vhost-user device must be able to handle both its own RX/TX
> virtqueues and the vhost-user virtqueues. So, the device must offer
> separate notification addresses and interrupt vectors for the vhost-user
> virtqueues. We are standardizing these resources with the
> VIRTIO_PCI_CAP_DOORBELL_CFG and VIRTIO_PCI_CAP_NOTIFICATION_CFG
> configuration structures.
> 
> > Can't we (ab)use the virtio pci registers for this?
> > Just extend VQ number to VQ/notification number.
> >
> 
> I am not sure I get your point. But, yes, I think we could use the
> existing registers (queue_select, queue_msix_vector, queue_notify_off)
> in the common configuration structure for both the device’s virtqueues
> and the vhost-user virtqueues. So, for example, we could use indexes 0,
> 1 for the device’s RX/TX virtqueues and index i+2 for the i-th
> vhost-user virtqueue.
> 
> However, I don't see why mixing up the configuration of the RX/TX and
> the vhost-user virtqueues is a better solution.

The reason is that this will automatically buy you support in all
transports: PCI/CCW/MMIO ...

> Wouldn’t it be more
> clean if we had separate configuration structures for the device's RX/TX
> virtqueues and the vhost-user virtqueues?

Flip this on its head and you will see that it buys you nothing except
cosmetics to separate them, and costs you portability across transports.


> --
> Nikos

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