linux-kernel - Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

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Date:   Thu, 24 Feb 2022 15:55:16 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

On Thu, Feb 24, 2022 at 03:51:56PM +0100, Peter Zijlstra wrote:
> +static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> +{
> +	u64 msr = CET_ENDBR_EN;
> +
> +	if (!HAS_KERNEL_IBT ||
> +	    !cpu_feature_enabled(X86_FEATURE_IBT))
> +		return;

If you add X86_FEATURE_BIT to arch/x86/include/asm/disabled-features.h,
the HAS_KERNEL_IBT check becomes redundant.

> +bool ibt_selftest(void)
> +{
> +	unsigned long ret;
> +
> +	asm ("1: lea 2f(%%rip), %%rax\n\t"
> +	     ANNOTATE_RETPOLINE_SAFE
> +	     "   jmp *%%rax\n\t"
> +	     ASM_REACHABLE
> +	     ANNOTATE_NOENDBR
> +	     "2: nop\n\t"
> +
> +	     /* unsigned ibt_selftest_ip = 2b */
> +	     ".pushsection .rodata,\"a\"\n\t"
> +	     ".align 8\n\t"
> +	     ".type ibt_selftest_ip, @object\n\t"
> +	     ".size ibt_selftest_ip, 8\n\t"
> +	     "ibt_selftest_ip:\n\t"
> +	     ".quad 2b\n\t"
> +	     ".popsection\n\t"

It's still seems silly to make this variable in asm.

Also .rodata isn't going to work for CPU hotplug.

-- 
Josh