linux-kernel - Re: [PATCH] x86/speculation: Simplify and make CALL

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Message-ID: <497a3694-cb0d-4678-9622-d9443bf8a40d@citrix.com>
Date: Thu, 27 Feb 2025 00:49:48 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org,
 Josh Poimboeuf <jpoimboe@...nel.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/speculation: Simplify and make CALL_NOSPEC consistent

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