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

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

On Fri, Apr 17, 2020 at 05:31:20PM +0800, Jason Wang wrote:
> 
> On 2020/4/17 下午2:39, Michael S. Tsirkin wrote:
> > 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.
> 
> 
> Yes, so as I replied in another thread. The doorbell of control vq should
> not share page with other doorbells.

Except page size can be as big as 64k on some systems.
The best thing is really if device just allows driver to write
anywhere within the page, taking VQ number from the data.


> Thanks
> 
> 
> > 
> > > > 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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> > > > 
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