RE: [virtio-comment] [PATCH v1 1/8] admin: Add theory of operation for device migration

[Parav Pandit <parav@nvidia.com>]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Wednesday, October 25, 2023 6:59 AM
> > For passthrough PASID assignment vq is not needed.
> 
> How do you know that? 
Because for passthrough, the hypervisor is not involved in dealing with VQ at all.

> There are works ongoing to make vPASID work for the
> guest like vSVA. Virtio doesn't differ from other devices.
Passthrough do not run like SVA. Each passthrough device has PASID from its own space fully managed by the guest.
Some cpu required vPASID and SIOV is not going this way anmore.

> 
> > If at all it is done, it will be done from the guest by the driver using virtio
> interface.
> 
> Then you need to trap. Such things couldn't be passed through to guests directly.
> 
Only PASID capability is trapped. PASID allocation and usage is directly from guest.
Regardless it is not relevant to passthrough mode as PASID is yet another resource.
And for some cpu if it is trapped, it is generic layer, that does not require virtio involvement.
So virtio interface asking to trap something because generic facility has done in not the approach.

> > Capabilities of #2 is generic across all pci devices, so it will be handled by the
> HV.
> > ATS/PRI cap is also generic manner handled by the HV and PCI device.
> 
> No, ATS/PRI requires the cooperation from the vIOMMU. You can simply do
> ATS/PRI passthrough but with an emulated vIOMMU.
And that is not the reason for virtio device to build trap+emulation for passthrough member devices.