Re: [virtio-dev] Re: [Virtio-networking] Doorbell mapping of vDPA

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

On Fri, Apr 17, 2020 at 12:22:04PM +0800, Jason Wang wrote:
> 
> On 2020/4/17 下午12:19, Jason Wang wrote:
> > 
> > On 2020/4/15 上午12:20, 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 we need intercept the doorbells that share a page with control vq
> doorbell.
> 
> Thanks


Which could be all of them. E.g. with a 4 byte offset, we are
talking 1K VQs per a 4k page.

> 
> > 
> > Or to facilitate the hardware design, we may introduce dedicated
> > notification area for control vq?
> > 
> > Thanks
> > 
> > 
> > > 
> > > 
> > > > > 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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> > > > > 
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> > 
> > 
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