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
