linux-kernel - Re: [PATCH v7 9/9] x86/startup_64: Drop global variables keeping track of LA57 state

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Date: Sat, 2 Mar 2024 00:55:14 +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>, 
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v7 9/9] x86/startup_64: Drop global variables keeping
 track of LA57 state

On Fri, 1 Mar 2024 at 20:20, Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Feb 27, 2024 at 04:19:17PM +0100, Ard Biesheuvel wrote:
> > +     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");
> > +
> > +     return ret;
>
> Yeah, I said this is creative but an alternative here looks like an
> overkill.
>
> Can we use a RIP_REL_REF(global var) instead pls?
>

I don't see the point of that, tbh. Patch #2 already ripped out all
the fixup_pointer() occurrences. This patch gets rid of the need to
#define USE_EARLY_PGTABLE_L5 in each translation unit that contains
code that might execute before alternatives patching has occurred.

Today, pgtable_l5_enabled() is used in many places, most of which
resolve to cpu_feature_enabled(), and I don't think you are suggesting
to replace all of those with a variable load, right? So that means
we'll have to stick with early and late variants of
pgtable_l5_enabled() like we have today, and we should just drop this
patch instead - I put it at the end of the series for a reason.