Re: [PATCH] x86/speculation: Simplify and make CALL_NOSPEC consistent

[Andrew Cooper]

On 26/02/2025 9:03 pm, Pawan Gupta wrote:
> @@ -420,20 +420,28 @@ static inline void call_depth_return_thunk(void) {}
>  
>  #ifdef CONFIG_X86_64
>  
> +/*
> + * Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call
> + * to the retpoline thunk with a CS prefix when the register requires
> + * a REX prefix byte to encode. Also see apply_retpolines().
> + */

Technically, both comments aren't quite accurate.  __CS_PREFIX() emits a
conditional CS prefix in a manner compatible with
-mindirect-branch-cs-prefix, not the full 5/6 byte jmp/call.

> +#define __CS_PREFIX(reg)				\
> +	.irp rs,r8,r9,r10,r11,r12,r13,r14,r15;		\
> +	.ifc \\rs, \reg;				\

Why are these escaped differently?  Given they're all \r of some form or
another, I guess something is going wonky with __stringify(), but its
still weird for them to be different.

Do you have a fully pre-processed source to hand to see how CALL_NOSPEC
ends up?

~Andrew