Re: [Virtio-networking] Doorbell mapping of vDPA

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

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.


> >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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