Re: [Virtio-networking] Doorbell mapping of vDPA

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

On Fri, Apr 17, 2020 at 12:19:43PM +0800, Jason Wang wrote:
> 
> On 2020/4/15 äå12:20, Michael S. Tsirkin wrote:
> > On Tue, Apr 14, 2020 at 01:12:51PM +0000, Vitaly Mireyno wrote:
> > > > -----Original Message-----
> > > > From: virtio-networking-bounces@redhat.com <virtio-networking-bounces@redhat.com> On Behalf
> > > > Of Jason Wang
> > > > Sent: Tuesday, 7 April, 2020 10:56
> > > > To: virtio-networking@redhat.com; Virtio-Dev <virtio-dev@lists.oasis-open.org>
> > > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > > Subject: [Virtio-networking] Doorbell mapping of vDPA
> > > > 
> > > > ----------------------------------------------------------------------
> > > > Hi all:
> > > > 
> > > > To get native performance of VF, we need to map doorbell to guest to avoid unnecessary vmexit. In
> > > > order to do this, we will launch qemu with page-per-vq=on. This means the each doorbell register
> > > > should be located at the beginning of 4K page and does not share the page with other registers. Then
> > > > vDPA framework can safely map it into the guest physical address (GPA) range defined by qemu. It
> > > > could be either
> > > > 
> > > > 1) a single doorbell register that is used by all virtqueues
> > > > 
> > > > or
> > > > 
> > > > 2) several different per-vq doorbell registers
> > > > 
> > > > If you decide to implement a virtio-pci register layout, need to make sure for notification structure
> > > > (4.1.4.4 of virtio spec):
> > > > 
> > > > For each virtqueue, the result
> > > > ofcap.offset+queue_notify_off*notify_off_multiplier is PAGE_SIZE (e.g
> > > > 4K) alignment, and the doorbeel does not share the page with other registers.
> > > > 
> > > > And it would be better if queue_notify_off, notify_off_multiplier can be changed via firmware for
> > > > extra flexibility.
> > > > 
> > > In some cases, these conditions could not be met for a virtio-net hardware device over PCI transport.
> > > queue_notify and notify_off_multiplier could not always be fully controlled by the firmware. There could be hardware limitations on flexibility degree of these parameters.
> > > Specifically, the limitations I'm thinking of are:
> > >   * queue_notify_off>0 and notify_off_multiplier>0
> > >   * Several doorbell registers of several virtqueues share the same page (but don't share the page with other registers).
> > > 
> > > Can this be supported in vDPA with direct doorbell mapping?
> > > 
> > > Thanks
> > There's value in being able to intercept some vqs in software
> > while the rest of vqs are handled in hardware.
> > E.g. that's the case for e.g. the control vq.
> 
> 
> Good point, so in this case, the doorbell of control vq must exclusively own
> a page.
> 
> Or to facilitate the hardware design, we may introduce dedicated
> notification area for control vq?
> 
> Thanks

Well all this would need spec changes. I'm guessing at this
point it's easier to just have hardware send commands to
qemu through some channel before using them. With SRIOV that would need
to be either the PF, or a memory bar of the VF.


> 
> > 
> > 
> > > > Please check and make sure your hardware have such ability and feel free to ask if you have questions
> > > > (offline if necessary).
> > > > 
> > > > Thanks
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
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