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

[Yu Zhang]

On Tue, Aug 31, 2021 at 09:53:27PM -0700, Andy Lutomirski wrote:
> 
> 
> 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.
> 

Thanks Andy. I was asking the first scenario.

Well, I agree it is doable if someone really want some assigned
device in TD guest. As Kevin mentioned in his reply, HPA can be
generated, by extending VFIO with a new mapping protocol which
uses fd+offset, instead of HVA. 

Another issue is current TDX does not support DMA encryption, and
only shared GPA memory shall be mapped in the VT-d. So to support
this, KVM may need to work with VFIO to dynamically program host
IOPT(IOMMU Page Table) when TD guest notifies a shared GFN range(e.g.,
with a MAP_GPA TDVMCALL), instead of prepopulating the IOPT at VM
creation time, by mapping entire GFN ranges of a guest.

So my inclination would be to just disallow using of VFIO device in
TDX first, until we have real requirement(with above enabling work
finished). 

B.R.
Yu