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Subject: Re: [Virtio-networking] Doorbell mapping of vDPA
On Fri, Apr 17, 2020 at 05:29:00PM +0800, Jason Wang wrote: > > On 2020/4/17 äå2:37, Michael S. Tsirkin wrote: > > 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. > > > Yes. > > > > I'm guessing at this > > point it's easier to just have hardware send commands to > > qemu through some channel before using them. > > > Yes, the plan is to let hardware report the location (offset) of the > doorbell registers. > > > > With SRIOV that would need > > to be either the PF, or a memory bar of the VF. > > > So I think what we want is: > > - map doorbells for non control vqs > - intercept the control vq doorbell > > In order to do this, hardware need to make sure that the doorbell of control > vq registers does not share page with any other doorbells. > > Then qemu may provide a suitable queue_notify_off for control vq which make > sure it can be intercepted independently. > > Thanks Right. So this is something we can consider adding to the spec: classification of VQs as data path/control, and then a recommendation to separate at least control from data. > > > > > > > > > > > > > > > Please check and make sure your hardware have such ability and feel free to ask if you have questions > > > > > > (offline if necessary). > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > > > > Virtio-networking mailing list > > > > > > Virtio-networking@redhat.com > > > > > > https://urldefense.proofpoint.com/v2/url?u=https-3A__www.redhat.com_mailman_listinfo_virtio- > > > > > > 2Dnetworking&d=DwIGaQ&c=nKjWec2b6R0mOyPaz7xtfQ&r=lDHJ2FW52oJ3lqqsArgFRdcevq01tbLQAw > > > > > > 4A_NO7xgI&m=pddgVCz- > > > > > > orGTuUXATJ4Dmi7vAXatG9w47AmULNC3V9A&s=5mfYWbLCjcZO8FcwDqgAc5bjE-H- > > > > > > 4p5TBkRZqP3uMsQ&e=
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