RE: [PATCH 1/6] x86/tdx: Support hypercalls for TDX guests on Hyper-V

[Dexuan Cui]

> From: Dave Hansen <dave.hansen@xxxxxxxxx>
> Sent: Monday, November 21, 2022 12:39 PM
> [...]
> On 11/21/22 11:51, Dexuan Cui wrote:
> > __tdx_hypercall() doesn't work for a TDX guest running on Hyper-V,
> > because Hyper-V uses a different calling convention, so add the
> > new function __tdx_ms_hv_hypercall().
> 
> Other than R10 being variable here and fixed for __tdx_hypercall(), this
> looks *EXACTLY* the same as __tdx_hypercall(), or at least a strict
> subset of what __tdx_hypercall() can do.
> 
> Did I miss something?

The existing asm code for __tdx_hypercall() passes through R10~R15
(see TDVMCALL_EXPOSE_REGS_MASK) to the (KVM) hypervisor.

Unluckily, for Hyper-V, we need to pass through RDX, R8, R10 and R11
to Hyper-V, so I don't think I can use the existing __tdx_hypercall() ?

> Another way of saying this:  It seems like you could do this with a new
> version of _tdx_hypercall() (and all in C) instead of a new
> __tdx_hypercall().

I don't think the current TDVMCALL_EXPOSE_REGS_MASK allows me
to pass through RDX and R8 to Hyper-V.

PS, the comment before __tdx_hypercall() contains this line:

"* RBX, RBP, RDI, RSI  - Used to pass VMCALL sub function specific
arguments."

But it looks like currently RBX an RBP are not used at all in 
arch/x86/coco/tdx/tdcall.S ?