linux-kernel - Re: [RFC PATCH 3/3] x86/boot: Use alternatives based selector for 5-level paging constants

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Message-ID: <aBo92LQh4WYXyRXK@gmail.com>
Date: Tue, 6 May 2025 18:50:32 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Ard Biesheuvel <ardb@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 3/3] x86/boot: Use alternatives based selector for
 5-level paging constants


* Ard Biesheuvel <ardb+git@...gle.com> wrote:

> +static inline int __attribute_const__ choose_l5_enabled(int yes, int no)
> +{
> +	int ret = no;
> +
> +	asm_inline(ALTERNATIVE_TERNARY("jmp 6f; 8:", %c[feat], "movl %[yes], %[ret]", "")
> +		"	.pushsection .altinstr_aux,\"ax\"	\n"
> +		"6:	pushfq					\n"
> +		"	testb	$1, %a[l5en]			\n"
> +		"	jz	7f				\n"
> +		"	movl	%[yes], %[ret]			\n"
> +		"7:	popfq					\n"
> +		"	jmp	8b				\n"
> +		"	.popsection				\n"
> +		: [ret]  "+rm" (ret)
> +		: [feat] "i" (X86_FEATURE_LA57),
> +		  [yes]	 "i" (yes),
> +		  [l5en] "i" (&__pgtable_l5_enabled));
> +
> +	return ret;

So why not create a new synthethic cpufeature flag, 
X86_FEATURE_LA57_ENABLED or so, which could then be queried via the 
regular facilities? This ternary logic is not really needed, because 
the hardware isn't ternary. :)

With that we could do with only a single, obvious line of ALTERNATIVE() 
assembly:

        #define ALTERNATIVES_CONST_U32(__val1, __val2, __feature)       \
        ({                                                              \
                u32 __val;                                              \
                                                                        \
                asm_inline (ALTERNATIVE("movl $" #__val1 ", %0", "movl $" __val2 ", %0", __feature) :"=g" (__val)); \
                                                                        \
                __val;                                                  \
        })

        ...

        #define MAX_PHYSMEM_BITS ALTERNATIVE_CONST_U32(46, 52, X86_FEATURE_LA57_ENABLED)

Thanks,

	Ingo