Re: [PATCH v2 00/11] Unify TDCALL/SEAMCALL and TDVMCALL assembly

[Peter Zijlstra]

On Fri, Jul 21, 2023 at 12:18:23AM +0000, Huang, Kai wrote:

> Unfortunately I don't think it's feasible.  Sean pointed out that
> kvm_vcpu_arch::regs[] do follow the "register index" hardware layout in x86 (for
> which I missed sorry), so we cannot re-order KVM part.  
> 
> And unfortunately RBP (5) is in middle of those registers:
> 
> 	0 = RAX
> 	1 = RCX
> 	2 = RDX
> 	3 = RBX
> 	4 = RSP
> 	5 = RBP
> 	6 = RSI
> 	7 = RDI
> 	8–15 represent R8–R15, respectively...
> 
> Thus unless we add RBP to 'struct tdx_module_args', it's impossible to re-order
> the structure to match KVM's layout.

Adding RBP to the struct shouldn't be a problem, we'll just not use it.
Same as RSP, nobody sane would expect that to be used. If you worry
about it you can give them funny names like:

struct tdx_module_args {
	unsigned long ax;
	unsigned long cx;
	unsigned long dx;
	unsigned long bx;
	unsigned long __sp_unused;
	unsigned long __bp_unused;
	unsigned long si;
	unsigned long di;
	...
};

I mean, at this point the whole thing is just 128 bytes of data anyway.