linux-kernel - Re: [PATCH v2 10/12] x86,nospec: Simplify {JMP,CALL}

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Message-ID: <20241115184056.ifkb6rhqpnucjn43@jpoimboe>
Date: Fri, 15 Nov 2024 10:40:56 -0800
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: seanjc@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
	jthoughton@...gle.com
Subject: Re: [PATCH v2 10/12] x86,nospec: Simplify {JMP,CALL}_NOSPEC (part 2)

On Mon, Nov 11, 2024 at 12:59:45PM +0100, Peter Zijlstra wrote:
> Counterpart to 09d09531a51a ("x86,nospec: Simplify
> {JMP,CALL}_NOSPEC"), x86_64 will rewrite all this anyway, see
> apply_retpoline.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/include/asm/nospec-branch.h |   29 +++++++++++------------------
>  1 file changed, 11 insertions(+), 18 deletions(-)
> 
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -429,31 +429,24 @@ static inline void call_depth_return_thu
>  
>  #ifdef CONFIG_X86_64
>  
> +#define __CS_PREFIX						\
> +	".irp rs,r8,r9,r10,r11,r12,r13,r14,r15\n"		\
> +	".ifc %V[thunk_target],\\rs\n"				\
> +	".byte 0x2e\n"						\
> +	".endif\n"						\
> +	".endr\n"

After staring at this for some minutes I'm thinking it would be helpful
to add a comment saying this is equivalent to
-mindirect-branch-cs-prefix.

-- 
Josh