Re: [RFC PATCH 12/21] KVM: IOMMUFD: MEMFD: Map private pages

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Aug 26, 2024 at 08:39:25AM +0000, Tian, Kevin wrote:
> > IOMMUFD calls get_user_pages() for every mapping which will allocate
> > shared memory instead of using private memory managed by the KVM and
> > MEMFD.
> > 
> > Add support for IOMMUFD fd to the VFIO KVM device's KVM_DEV_VFIO_FILE
> > API
> > similar to already existing VFIO device and VFIO group fds.
> > This addition registers the KVM in IOMMUFD with a callback to get a pfn
> > for guest private memory for mapping it later in the IOMMU.
> > No callback for free as it is generic folio_put() for now.
> > 
> > The aforementioned callback uses uptr to calculate the offset into
> > the KVM memory slot and find private backing pfn, copies
> > kvm_gmem_get_pfn() pretty much.
> > 
> > This relies on private pages to be pinned beforehand.
> > 
> 
> There was a related discussion [1] which leans toward the conclusion
> that the IOMMU page table for private memory will be managed by
> the secure world i.e. the KVM path.

It is still effectively true, AMD's design has duplication, the RMP
table has the mappings to validate GPA and that is all managed in the
secure world.

They just want another copy of that information in the unsecure world
in the form of page tables :\

> btw going down this path it's clearer to extend the MAP_DMA
> uAPI to accept {gmemfd, offset} than adding a callback to KVM.

Yes, we want a DMA MAP from memfd sort of API in general. So it should
go directly to guest memfd with no kvm entanglement.

Jason