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

[Dave Hansen]

On 11/22/22 17:37, Dexuan Cui wrote:
>> 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() ?

What's to prevent you from adding RDX and R8?  You could make
TDVMCALL_EXPOSE_REGS_MASK a macro argument.

Look at 'has_erro_code', for instance in "idtentry_body"
arch/x86/entry/entry_64.S.

>> 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.

Right.  So pass it in.

> 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 ?

Yeah, it looks like they are a part of the hypercall ABI but no existing
hypercall is using them.  Patches to fix it accepted. :)