RE: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Tuesday, November 10, 2020 10:24 PM
> 
> On Tue, Nov 10, 2020 at 06:13:23AM -0800, Raj, Ashok wrote:
> 
> > This isn't just for idxd, as I mentioned earlier, there are vendors other
> > than Intel already working on this. In all cases the need for guest direct
> > manipulation of interrupt store hasn't come up. From the discussion, it
> > seems like there are devices today or in future that will require direct
> > manipulation of interrupt store in the guest. This needs additional work
> > in both the device hardware providing the right plumbing and OS work to
> > comprehend those.
> 
> We'd want to see SRIOV's assigned to guests to be able to use
> IMS. This allows a SRIOV instance in a guest to spawn SIOV's which is
> useful.

Does your VF support both MSI/IMS or IMS only? If it is the former can't
we adopt a phased approach or parallel effort between forcing guest
to use MSI and adding hypercall to enable IMS on VF? Finding a way
to disable IMS is anyway required per earlier discussion when hypercall
is not available, and it could still provide a functional though suboptimal
model for such VFs.

> 
> SIOV's assigned to guests could use IMS, but the use cases we see in
> the short term can be handled by using SRIOV instead.
> 
> I would expect in general for SIOV to use MSI-X emulation to expose
> interrupts - it would be really weird for a SIOV emulator to do
> something else and we should probably discourage that.
> 

I agree with this point. This leaves hardware gaps in IOMMU and root
complex less an immediate blocker and to be addressed in the long term.

Thanks
Kevin