RE: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

From: Tian, Kevin
Date: Wed Sep 01 2021 - 03:12:43 EST

> From: Andy Lutomirski <luto@xxxxxxxxxx>
> Sent: Wednesday, September 1, 2021 12:53 PM
> On Thu, Aug 26, 2021, at 7:31 PM, Yu Zhang wrote:
> > On Thu, Aug 26, 2021 at 12:15:48PM +0200, David Hildenbrand wrote:
> > Thanks a lot for this summary. A question about the requirement: do we or
> > do we not have plan to support assigned device to the protected VM?
> >
> > If yes. The fd based solution may need change the VFIO interface as well(
> > though the fake swap entry solution need mess with VFIO too). Because:
> >
> > 1> KVM uses VFIO when assigning devices into a VM.
> >
> > 2> Not knowing which GPA ranges may be used by the VM as DMA buffer,
> all
> > guest pages will have to be mapped in host IOMMU page table to host
> pages,
> > which are pinned during the whole life cycle fo the VM.
> >
> > 3> IOMMU mapping is done during VM creation time by VFIO and IOMMU
> driver,
> > in vfio_dma_do_map().
> >
> > 4> However, vfio_dma_do_map() needs the HVA to perform a GUP to get
> the HPA
> > and pin the page.
> >
> > But if we are using fd based solution, not every GPA can have a HVA, thus
> > the current VFIO interface to map and pin the GPA(IOVA) wont work. And I
> > doubt if VFIO can be modified to support this easily.
> >
> >
> Do you mean assigning a normal device to a protected VM or a hypothetical
> protected-MMIO device?
> If the former, it should work more or less like with a non-protected VM.
> mmap the VFIO device, set up a memslot, and use it. I'm not sure whether
> anyone will actually do this, but it should be possible, at least in principle.
> Maybe someone will want to assign a NIC to a TDX guest. An NVMe device
> with the understanding that the guest can't trust it wouldn't be entirely crazy
> ether.
> If the latter, AFAIK there is no spec for how it would work even in principle.
> Presumably it wouldn't work quite like VFIO -- instead, the kernel could have
> a protection-virtual-io-fd mechanism, and that fd could be bound to a
> memslot in whatever way we settle on for binding secure memory to a
> memslot.

FYI the iommu logic in VFIO is being refactored out into an unified /dev/iommu
framework [1]. Currently it plans to support the same DMA mapping semantics
as what VFIO provides today (HVA-based). in the future it could be extended
to support another mapping protocol which accepts fd+offset instead of HVA
and then calls the helper function from whatever backing store which can help
translate fd+offset to HPA instead of using GUP.