Re: [PATCH v3 00/21] TDX host kernel support

[Dave Hansen]

On 4/29/22 10:48, Dan Williams wrote:
>> But, neither of those really help with, say, a device-DAX mapping of
>> TDX-*IN*capable memory handed to KVM.  The "new syscall" would just
>> throw up its hands and leave users with the same result: TDX can't be
>> used.  The new sysfs ABI for NUMA nodes wouldn't clearly apply to
>> device-DAX because they don't respect the NUMA policy ABI.
> They do have "target_node" attributes to associate node specific
> metadata, and could certainly express target_node capabilities in its
> own ABI. Then it's just a matter of making pfn_to_nid() do the right
> thing so KVM kernel side can validate the capabilities of all inbound
> pfns.

Let's walk through how this would work with today's kernel on tomorrow's
hardware, without KVM validating PFNs:

1. daxaddr mmap("/dev/dax1234")
2. kvmfd = open("/dev/kvm")
3. ioctl(KVM_SET_USER_MEMORY_REGION, { daxaddr };
4. guest starts running
5. guest touches 'daxaddr'
6. Page fault handler maps 'daxaddr'
7. KVM finds new 'daxaddr' PTE
8. TDX code tries to add physical address to Secure-EPT
9. TDX "SEAMCALL" fails because page is not convertible
10. Guest dies

All we can do to improve on that is call something that pledges to only
map convertible memory at 'daxaddr'.  We can't *actually* validate the
physical addresses at mmap() time or even
KVM_SET_USER_MEMORY_REGION-time because the memory might not have been
allocated.

Those pledges are hard for anonymous memory though.  To fulfill the
pledge, we not only have to validate that the NUMA policy is compatible
at KVM_SET_USER_MEMORY_REGION, we also need to decline changes to the
policy that might undermine the pledge.