linux-kernel - Re: [PATCH] x86/alternative: Optimize returns patching

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Date:   Fri, 12 May 2023 14:49:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/alternative: Optimize returns patching

On Fri, May 12, 2023 at 02:09:52PM +0200, Borislav Petkov wrote:
> From: "Borislav Petkov (AMD)" <bp@...en8.de>
> 
> Instead of decoding each instruction in the return sites range only to
> realize that that return site is a jump to the default return thunk
> which is needed - X86_FEATURE_RETHUNK is enabled - lift that check
> before the loop and get rid of that loop overhead.
> 
> Add comments about what gets patched, while at it.
> 
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> ---
>  arch/x86/kernel/alternative.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index b78d55f0dfad..3bb0a5f61e8c 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -693,13 +693,12 @@ static int patch_return(void *addr, struct insn *insn, u8 *bytes)
>  {
>  	int i = 0;
>  
> +	/* Patch the custom return thunks... */
>  	if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {
> -		if (x86_return_thunk == __x86_return_thunk)
> -			return -1;
> -
>  		i = JMP32_INSN_SIZE;
>  		__text_gen_insn(bytes, JMP32_INSN_OPCODE, addr, x86_return_thunk, i);
>  	} else {
> +		/* ... or patch them out if not needed. */
>  		bytes[i++] = RET_INSN_OPCODE;
>  	}
>  
> @@ -712,6 +711,14 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end)
>  {
>  	s32 *s;
>  
> +	/*
> +	 * Do not patch out the default return thunks if those needed are the
> +	 * ones generated by the compiler.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_RETHUNK) &&
> +	    (x86_return_thunk == __x86_return_thunk))
> +		return;
> +
>  	for (s = start; s < end; s++) {
>  		void *dest = NULL, *addr = (void *)s + *s;
>  		struct insn insn;
> -- 
> 2.35.1
>