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

[Dexuan Cui <decui@microsoft.com>]

> From: Dave Hansen <dave.hansen@intel.com>
> Sent: Monday, November 28, 2022 7:22 AM
> [...]
> On 11/27/22 16:58, Dexuan Cui wrote:
> > +u64 hv_tdx_hypercall(u64 control, u64 input_addr, u64 output_addr)
> > +{
> > +	struct tdx_hypercall_args args = { };
> > +
> > +	if (!(control & HV_HYPERCALL_FAST_BIT)) {
> > +		if (input_addr)
> > +			input_addr += ms_hyperv.shared_gpa_boundary;
> > +
> > +		if (output_addr)
> > +			output_addr += ms_hyperv.shared_gpa_boundary;
> > +	}
> 
> This:
>  [...]
> makes it sound like HV_HYPERCALL_FAST_BIT says whether arguments go in
> registers (fast) or memory (slow).  But this hv_tdx_hypercall() function
> looks like it takes addresses only.

Good point! When hv_tdx_hypercall() is called from hv_do_fast_hypercall8()
and hv_do_fast_hypercall16(), actually the two parameters are not memory
addresses. I'll rename the parameters to param1 and param2. 

I also realized I need to export the function for drivers. 

> *Is* there a register based calling convention to make Hyper-V
> hypercalls when running under TDX?

When hv_tdx_hypercall() is called from hv_do_fast_hypercall8()
and hv_do_fast_hypercall16(), the params are indeed passed via registers
rather than memory.

> Also, is this the output address manipulation fundamentally different from:
> 
> 	output_addr = cc_mkdec(output_addr);
> 
> ?  Decrypted addresses are the shared ones, right?
Yes. 

Good point -- I'll use the updated version:

u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2)
{
        struct tdx_hypercall_args args = { };

        if (!(control & HV_HYPERCALL_FAST_BIT)) {
                if (param1)
                        param1 = cc_mkdec(param1);

                if (param2)
                        param2 = cc_mkdec(param2);
        }

        args.r10 = control;
        args.rdx = param1;
        args.r8  = param2;

        (void)__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT);

        return args.r11;
}
EXPORT_SYMBOL_GPL(hv_tdx_hypercall);