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From: Ard Biesheuvel <ardb@kernel.org>
Date: Sun, 11 Feb 2024 23:36:51 +0100
Message-ID: <CAMj1kXG5+KEAE5F0EFkZCkt1244M3UBJXH_O3ULg0oRHV2YnmQ@mail.gmail.com>
Subject: Re: [PATCH v3 06/19] x86/startup_64: Drop global variables keeping
 track of LA57 state
To: Borislav Petkov <bp@alien8.de>
Cc: Ard Biesheuvel <ardb+git@google.com>, linux-kernel@vger.kernel.org, 
	Kevin Loughlin <kevinloughlin@google.com>, Tom Lendacky <thomas.lendacky@amd.com>, 
	Dionna Glaze <dionnaglaze@google.com>, Thomas Gleixner <tglx@linutronix.de>, 
	Ingo Molnar <mingo@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, 
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>, Nathan Chancellor <nathan@kernel.org>, 
	Nick Desaulniers <ndesaulniers@google.com>, Justin Stitt <justinstitt@google.com>, 
	Kees Cook <keescook@chromium.org>, Brian Gerst <brgerst@gmail.com>, linux-arch@vger.kernel.org, 
	llvm@lists.linux.dev
Content-Type: text/plain; charset="UTF-8"

On Sat, 10 Feb 2024 at 11:41, Borislav Petkov <bp@alien8.de> wrote:
>
> 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 ...
>

We can actually rip all of that stuff out, and have only a single
implementation of pgtable_l5_enabled() that is not based on a variable
at all. It results in a nice cleanup, but I'll keep it as a separate
patch in the next revision so we can easily drop it if preferred.