Re: [PATCH v2 17/19] iommu/arm-smmu-v3: Add arm_smmu_viommu_cache_invalidate

Jason Gunthorpe
2024-09-23T18:36:26Z

On Wed, Sep 18, 2024 at 08:10:52AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Saturday, September 14, 2024 10:51 PM
> >
> > On Fri, Sep 13, 2024 at 02:33:59AM +0000, Tian, Kevin wrote:
> > > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > > Sent: Thursday, September 12, 2024 7:08 AM
> > > >
> > > > On Wed, Sep 11, 2024 at 08:13:01AM +0000, Tian, Kevin wrote:
> > > >
> > > > > Probably there is a good reason e.g. for simplification or better
> > > > > aligned with hw accel stuff. But it's not explained clearly so far.
> > > >
> > > > Probably the most concrete thing is if you have a direct assignment
> > > > invalidation queue (ie DMA'd directly by HW) then it only applies to a
> > > > single pIOMMU and invalidation commands placed there are unavoidably
> > > > limited in scope.
> > > >
> > > > This creates a representation problem, if we have a vIOMMU that spans
> > > > many pIOMMUs but invalidations do some subset how to do we model
> > > > that. Just saying the vIOMMU is linked to the pIOMMU solves this
> > > > nicely.
> > > >
> > >
> > > yes that is a good reason.
> > >
> > > btw do we expect the VMM to try-and-fail when deciding whether a
> > > new vIOMMU object is required when creating a new vdev?
> >
> > I think there was some suggestion the getinfo could return this, but
> > also I think qemu needs to have a command line that matches physical
> > so maybe it needs some sysfs?
> >
>
> My impression was that Qemu is moving away from directly accessing
> sysfs (e.g. as the reason behind allowing Libvirt to pass in an opened
> cdev fd to Qemu). So probably getinfo makes more sense...

Yes, but I think libvirt needs this information before it invokes
qemu..

The physical and virtual iommus need to sort of match, something
should figure this out automatically I would guess.

Jason