Re: [PATCH 03/20] x86/tdx: Convert port I/O handling to use new TDVMCALL macros

Paolo Bonzini
2024-05-17T17:37:43Z

On 5/17/24 17:28, Dave Hansen wrote:
> On 5/17/24 07:19, Kirill A. Shutemov wrote:
>> static inline void tdx_io_out(int size, u16 port, u32 value)
>> {
>> - struct tdx_module_args args = {
>> - .r10 = TDX_HYPERCALL_STANDARD,
>> - .r11 = hcall_func(EXIT_REASON_IO_INSTRUCTION),
>> - .r12 = size,
>> - .r13 = 1,
>> - .r14 = port,
>> - .r15 = value,
>> - };
>> -
>> - __tdx_hypercall(&args);
>> + TDVMCALL_0(hcall_func(EXIT_REASON_IO_INSTRUCTION),
>> + size, TDX_PORT_WRITE, port, value);
>> }
>
> I actually really like the self-documenting nature of the structures. I
> don't think it's a win if this is where the lines-of-code savings comes
> from.
>

It's just a tradeoff. For example someone could well have written

#define TDVMCALL_0(reason, a1, a2, a3, a4) \
do { \
struct tdx_module_args args = {
.r10 = TDX_HYPERCALL_STANDARD,
.r11 = reason,
.r12 = a1,
.r13 = a2,
.r14 = a3,
.r15 = a4,
__tdx_hypercall(&args);
} while(0)

even with the current __tdx_hypercall() implementation.

I agree that TDVMCALL_x is somewhat less legible; on the other hand it
highlights that these TDVMCALLs all have a common convention for passing
parameters / retrieving results, and reduces the potential for silly typos.

This is also why I asked about the different approaches for TDCALL vs.
TDVMCALL. Given that there are only a handful of appearances for
tdvmcall_trampoline, maybe the best of both worlds is just to inline the
whole thing? This way the code in the macros matches the parameter
passing convention of the GHCI.

Paolo