RE: [patch V2 31/33] iommu/vt-d: Enable PCI/IMS

[Tian, Kevin]

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Thursday, November 24, 2022 9:21 PM
> 
> On Thu, Nov 24 2022 at 09:14, Jason Gunthorpe wrote:
> > On Thu, Nov 24, 2022 at 10:37:53AM +0100, Thomas Gleixner wrote:
> >> Jason said, that the envisioned Mellanox use case does not depend on the
> >> IOMMU because the card itself has one which takes care of the
> >> protections.
> >
> > Right, but that doesn't mean we need the physical iommu turned
> > off. Setting the mlx pci device to identity mode is usually enough to
> > get back to full performance.
> 
> Ok.

yes. IR can be enabled orthogonal to DMA mapping mode in IOMMU.

> 
> >> How are we going to resolve that dilemma?
> >
> > The outcome is we don't have a strategy right now to make IMS work in
> > VMs. This series is all about making it work on physical machines,
> > that has to be a good first step.

yes, that is the point. As long as IMS is disabled in the guest it's already
a good first step for the moment.

> >
> > I'm hoping the OCP work stream on SIOV will tackle how to fix the
> > interrupt problems. Some of the ideas I've seen could be formed into
> > something that would work in a VM.
> 
> Fair enough.
> 
> Let me put the limitation into effect then.
> 
> Thanks,
> 
>         tglx