Re: [virtio-comment] RE: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ

[Jason Wang <jasowang@redhat.com>]

å 2023/5/11 22:31, Parav Pandit åé:
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Thursday, May 11, 2023 3:17 AM
> > > a configq/cmdq discussion is for new features for PF, VF, and SF/SIOV
> > > devices to work in _non_ backward compatible way, because for new
> > > features there is no such thing as backward compatibility between
> > > guest driver and the device.
> > Ok, so what command did configq/cmdq have?
> > I thought it was similar to transport virtqueue, but it looks more like a control virtqueue here.
> Yes, it is more like a cvq to do device attributes discovery and configuration.
> Just that its generic enough.
> For example
> creating VQ that spans multiple physical addresses.
> Discovery and enabling new device attributes which are rarely accessed or accessed during device init/cleanup time.
>
> The fundamental difference between cfgvq to tvq is:
> Cfgvq is between the member virtio device and its driver.
> Tvq=aq is on group owner of the virtio device.
>
> > > > > Compatibility is coming at a cost which is not scaling.
> > > > > If you attempt to scale, it breaks the future path forward to go without
> > mediation.
> > > > > > > So eve growing these fields and optionally placement on configq
> > > > > > > doesn't really help and device builder to build it efficiently
> > > > > > > (without much predictability).
> > > > > > Config queue is not the only choice, we have a lot of other
> > > > > > choices (for example PASID may help to reduce the on-chip resources).
> > > > > PASID is for process isolation security construct, it is not for transportation.
> > > > I meant with PASID you don't even need a BAR.
> > > Not sure I fully understand, but my guess is, this is coming from
> > > mediation thought process.
> > Not actually. I meant, for example without PASID, for modern devices, the
> > common cfg structure must be placed on a BAR through MMIO.
> >
> > But with the help of PASID, it could stay in the memory and the device can fetch
> > it via DMA. The help to reduce the resources for registers.
> Yes, it can stay in memory and device can fetch via DMA using a VQ using its own q from guest driver.
> Why does it need PASID?


Let me clarify my understanding.

If we want a per VF virtqueue that is used by the hypervisor. We need 
PASID. Otherwise not.

Thanks