linux-kernel - Re: [PATCH 4/9] x86/alternative: Implement .retpoline

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Message-ID: <20211013203927.zbruy2ux7dug6ljk@treble>
Date:   Wed, 13 Oct 2021 13:39:27 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

On Wed, Oct 13, 2021 at 02:22:21PM +0200, Peter Zijlstra wrote:
> +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> +{
> +	void (*target)(void);
> +	int reg, i = 0;
> +
> +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> +		return -1;

Better to do this check further up the call stack in apply_retpolines()
before looping through all the call sites?

> +
> +	target = addr + insn->length + insn->immediate.value;
> +	reg = (target - &__x86_indirect_thunk_rax) /
> +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> +
> +	if (WARN_ON_ONCE(reg & ~0xf))
> +		return -1;

It would be more robust and less magical to just have a basic lookup
table array which converts a thunk address to a reg.  Then you can just
avoid all the safety checks because it's no longer insane ;-)

-- 
Josh