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Date:   Mon, 21 Mar 2022 11:07:01 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
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Subject: Re: [PATCH v4 14/32] iommu: introduce iommu_domain_alloc_type and
 the KVM type
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On Sat, Mar 19, 2022 at 07:51:31AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Friday, March 18, 2022 10:13 PM
> > 
> > On Fri, Mar 18, 2022 at 02:23:57AM +0000, Tian, Kevin wrote:
> > 
> > > Yes, that is another major part work besides the iommufd work. And
> > > it is not compatible with KVM features which rely on the dynamic
> > > manner of EPT. Though It is a bit questionable whether it's worthy of
> > > doing so just for saving memory footprint while losing other capabilities,
> > > it is a requirement for some future security extension in Intel trusted
> > > computing architecture. And KVM has been pinning pages for SEV/TDX/etc.
> > > today thus some facilities can be reused. But I agree it is not a simple
> > > task thus we need start discussion early to explore various gaps in
> > > iommu and kvm.
> > 
> > Yikes. IMHO this might work better going the other way, have KVM
> > import the iommu_domain and use that as the KVM page table than vice
> > versa.
> > 
> > The semantics are a heck of a lot clearer, and it is really obvious
> > that alot of KVM becomes disabled if you do this.
> > 
> 
> This is an interesting angle to look at it. But given pinning is already
> required in KVM to support SEV/TDX even w/o assigned device, those
> restrictions have to be understood by KVM MMU code which makes
> a KVM-managed page table under such restrictions closer to be 
> sharable with IOMMU.

I thought the SEV/TDX stuff wasn't being done with pinning but via a
memfd in a special mode that does sort of pin under the covers, but it
is not necessarily a DMA pin. (it isn't even struct page memory, so
I'm not even sure what pin means)

Certainly, there is no inherent problem with SEV/TDX having movable
memory and KVM could concievably handle this - but iommu cannot.

I would not make an equivilance with SEV/TDX and iommu at least..

Jason