RE: [RFC] /dev/ioasid uAPI proposal

[Tian, Kevin]

> From: David Gibson
> Sent: Thursday, June 3, 2021 1:09 PM
[...]
> > > In this way the SW mode is the same as a HW mode with an infinite
> > > cache.
> > >
> > > The collaposed shadow page table is really just a cache.
> > >
> >
> > OK. One additional thing is that we may need a 'caching_mode"
> > thing reported by /dev/ioasid, indicating whether invalidation is
> > required when changing non-present to present. For hardware
> > nesting it's not reported as the hardware IOMMU will walk the
> > guest page table in cases of iotlb miss. For software nesting
> > caching_mode is reported so the user must issue invalidation
> > upon any change in guest page table so the kernel can update
> > the shadow page table timely.
> 
> For the fist cut, I'd have the API assume that invalidates are
> *always* required.  Some bypass to avoid them in cases where they're
> not needed can be an additional extension.
> 

Isn't a typical TLB semantics is that non-present entries are not
cached thus invalidation is not required when making non-present
to present? It's true to both CPU TLB and IOMMU TLB. In reality
I feel there are more usages built on hardware nesting than software
nesting thus making default following hardware TLB behavior makes
more sense...

Thanks
Kevin