Re: [PATCH] x86/tdx: Enhance code generation for TDCALL and SEAMCALL wrappers

[Dave Hansen <dave.hansen@intel.com>]

On 6/2/24 04:54, Kirill A. Shutemov wrote:
> Sean observed that the compiler is generating inefficient code to clear
> the tdx_module_args struct for TDCALL and SEAMCALL wrappers. The
> compiler is generating numerous instructions at each call site to clear
> the unused fields of the structure.
> 
> To address this issue, avoid using C99-initializer and instead
> explicitly use string instructions to clear the struct.
> 
> With Clang, this change results in a savings of approximately 3K with my
> configuration:
> 
> add/remove: 0/0 grow/shrink: 0/21 up/down: 0/-3187 (-3187)
> 
> With GCC, the savings are less significant at around 300 bytes:
> 
> add/remove: 0/0 grow/shrink: 3/22 up/down: 17/-313 (-296)
> 
> GCC tends to generate string instructions more frequently to clear the
> struct.

<shrug>

I don't think moving away from perfectly normal C struct initialization
is worth it for 300 bytes of text in couple of slow paths.

If someone out there is using clang, is confident that it is doing the
right thing and not just being silly, _and_ is horribly bothered by its
code generation, then please speak up.

> +static __always_inline void tdx_arg_init(struct tdx_module_args *args)
> +{
> +	asm ("rep stosb"
> +	     : "+D" (args)
> +	     : "c" (sizeof(*args)), "a" (0)
> +	     : "memory");
> +}

The inline assembly also has the side-effect of tripping up the
compiler.  The compiler can't optimize across these at all and it
probably has the effect of bloating the code.

Oh, and if we're going to leave this weirdo initialization idiom for
TDX, it needs to be well commented:

/*
 * Using normal " = {};" to initialize tdx_module_args results in
 * bloated hard-to-read assembly.  Zero it using the most compact way
 * available.
 */

Eh?