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

[Andy Lutomirski]

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.