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

[Sathyanarayanan Kuppuswamy]

On 11/22/22 5:37 PM, 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() ?
> >> 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.

Because TDVMCALLs defined in the GHCI specification only use registers
R10-R15, only those registers are currently exposed. However, the TDVMCALL
ABI allows the use of input registers such as RBX, RBP, RDI, RSI, R8 or R9.
Instead of creating a new variant of __tdx_hypercall() to handle your use
case, perhaps you can add the registers you require to the
TDVMCALL EXPOSE REGS MASK. You just need to make sure they are zeroed out
for other users of __tdx_hypercall().

> 
> 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 ?
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer