Re: [PATCH 5/6] x86/hyperv: Support hypercalls for TDX guests

[Dave Hansen]

On 11/28/22 11:37, Dexuan Cui wrote:
>> From: Dave Hansen <dave.hansen@xxxxxxxxx>
...
>> How do we know, for instance, that no hypercall using this interface
>> will *ever* take the 0x0 physical address as an argument?
> 
> A 0x0 physical address as an argument still works: the 0 is passed
> to the hypervisor using GHCI. I believe Hyper-V interprets the 0 as
> an error (if the param is needed), and returns an "invalid parameter"
> error code to the guest.

I don't see any data in the public documentation to support the claim
that 0x0 is a special argument for either the input or output GPA
parameters.

This is despite some actual discussion on things like their alignment
requirements[1] and interactions with overlay pages.

So, either you are mistaken about that behavior, or it looks like the
documentation needs updating.

1.
https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/hypercall-interface