linux-kernel - Re: [PATCH v3 06/19] x86/startup_64: Drop global variables keeping track of LA57 state

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Date: Sat, 10 Feb 2024 11:40:39 +0100
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, 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 v3 06/19] x86/startup_64: Drop global variables keeping
 track of LA57 state

On Fri, Feb 09, 2024 at 01:55:02PM +0000, Ard Biesheuvel wrote:
> I was trying to get rid of global variable assignments and accesses
> from the 1:1 mapping, but since we cannot get rid of those entirely,
> we might just keep __pgtable_l5_enabled but use RIP_REL_REF() in the
> accessors, and move the assignment to the asm startup code.

Yeah.

>    asm(ALTERNATIVE_TERNARY(
>        "movq %%cr4, %[reg] \n\t btl %[la57], %k[reg]" CC_SET(c),
>        %P[feat], "stc", "clc")
>        : [reg] "=r" (r), CC_OUT(c) (ret)
>        : [feat] "i" (X86_FEATURE_LA57),
>          [la57] "i" (X86_CR4_LA57_BIT)
>        : "cc");

Creative :)

> but we'd still have two versions in that case.

Yap. RIP_REL_REF() ain't too bad ...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette