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

[Yu Zhang]

On Wed, Sep 01, 2021 at 06:27:20PM +0200, David Hildenbrand wrote:
> On 01.09.21 18:07, Andy Lutomirski wrote:
> > On 9/1/21 3:24 AM, Yu Zhang wrote:
> > > 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.
> > 
> > I'm confused.  I don't see why any new code is needed for this at all.
> > Every proposal I've seen for handling TDX memory continues to handle TDX
> > *shared* memory exactly like regular guest memory today.  The only
> > differences are that more hole punching will be needed, which will
> > require lightweight memslots (to have many of them), memslots with
> > holes, or mappings backing memslots with holes (which can be done with
> > munmap() on current kernels).
> > 
> > So you can literally just mmap a VFIO device and expect it to work,
> > exactly like it does right now.  Whether the guest will be willing to
> > use the device will depend on the guest security policy (all kinds of
> > patches about that are flying around), but if the guest tries to use it,
> > it really should just work.
> 
> ... but if you end up mapping private memory into IOMMU of the device and
> the device ends up accessing that memory, we're in the same position that
> the host might get capped, just like access from user space, no?

Well, according to the spec:

  - If the result of the translation results in a physical address with a TD
  private key ID, then the IOMMU will abort the transaction and report a VT-d
  DMA remapping failure.

  - If the GPA in the transaction that is input to the IOMMU is private (SHARED
  bit is 0), then the IOMMU may abort the transaction and report a VT-d DMA
  remapping failure.

So I guess mapping private GPAs in IOMMU is not that dangerous as mapping
into userspace. Though still wrong.

> 
> Sure, you can map only the complete duplicate shared-memory region into the
> IOMMU of the device, that would work. Shame vfio mostly always pins all
> guest memory and you essentially cannot punch holes into the shared memory
> anymore -- resulting in the worst case in a duplicate memory consumption for
> your VM.
> 
> So you'd actually want to map only the *currently* shared pieces into the
> IOMMU and update the mappings on demand. Having worked on something related,

Exactly. On demand mapping and page pinning for shared memory is necessary.

> I can only say that 64k individual mappings, and not being able to modify
> existing mappings except completely deleting them to replace with something
> new (!atomic), can be quite an issue for bigger VMs.

Do you mean atomicity in mapping/unmapping can hardly be guaranteed during
the shared <-> private transition? May I ask for elaboration? Thanks!

B.R.
Yu