linux-kernel - Re: [PATCH v4 04/11] x86/startup_64: Defer assignment of 5-level paging global variables

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Date: Wed, 21 Feb 2024 11:20:13 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, Kevin Loughlin <kevinloughlin@...gle.com>, 
	Tom Lendacky <thomas.lendacky@....com>, Dionna Glaze <dionnaglaze@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Brian Gerst <brgerst@...il.com>, linux-arch@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: Re: [PATCH v4 04/11] x86/startup_64: Defer assignment of 5-level
 paging global variables

On Wed, 21 Feb 2024 at 11:09, Borislav Petkov <bp@...en8.de> wrote:
>
> On Wed, Feb 21, 2024 at 12:33:08AM +0100, Ard Biesheuvel wrote:
> > Right, this is the same issue as in #11 - in both cases, the extern
> > declaration of __pgtable_l5_enabled needs to be visible regardless of
> > CONFIG_X86_5LEVEL.
>
> Yap, I don't mind something like below.
>
> 5LEVEL will be practically enabled everywhere.
>
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 158da0fd01d2..eeb1744215f2 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -52,13 +52,11 @@ extern pmd_t early_dynamic_pgts[EARLY_DYNAMIC_PAGE_TABLES][PTRS_PER_PMD];
>  static unsigned int __initdata next_early_pgt;
>  pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | _PAGE_NX);
>
> -#ifdef CONFIG_X86_5LEVEL
>  unsigned int __pgtable_l5_enabled __ro_after_init;
>  unsigned int pgdir_shift __ro_after_init = 39;
>  EXPORT_SYMBOL(pgdir_shift);
>  unsigned int ptrs_per_p4d __ro_after_init = 1;
>  EXPORT_SYMBOL(ptrs_per_p4d);
> -#endif
>

Just the below should be sufficient

--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -22,7 +22,7 @@ typedef struct { pteval_t pte; } pte_t;
 typedef struct { pmdval_t pmd; } pmd_t;

-#ifdef CONFIG_X86_5LEVEL
 extern unsigned int __pgtable_l5_enabled;

+#ifdef CONFIG_X86_5LEVEL
 #ifdef USE_EARLY_PGTABLE_L5
 /*