linux-kernel - RE: [PATCH v4 14/32] iommu: introduce iommu_domain_alloc

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Date:   Sat, 19 Mar 2022 07:51:31 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Robin Murphy <robin.murphy@....com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "david@...hat.com" <david@...hat.com>,
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        Jean-Philippe Brucker <jean-philippe@...aro.org>,
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        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: RE: [PATCH v4 14/32] iommu: introduce iommu_domain_alloc_type and the
 KVM type

> From: Jason Gunthorpe <jgg@...dia.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.

Thanks
Kevin