linux-kernel - Re: [PATCH v3 05/19] x86/startup_64: Simplify CR4 handling in startup code

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Date: Wed, 7 Feb 2024 10:38:20 +0000
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
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 05/19] x86/startup_64: Simplify CR4 handling in startup code

On Tue, 6 Feb 2024 at 18:21, Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Jan 29, 2024 at 07:05:08PM +0100, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > When executing in long mode, the CR4.PAE and CR4.LA57 control bits
> > cannot be updated,
>
> "Long mode requires PAE to be enabled in order to use the 64-bit
> page-translation data structures to translate 64-bit virtual addresses
> to 52-bit physical addresses."
>
> which is actually already enabled at that point:
>
> cr4            0x20                [ PAE ]
>
> "5-Level paging is enabled by setting CR4[LA57]=1 when EFER[LMA]=1.
> CR4[LA57] is ignored when long mode is not active (EFER[LMA]=0)."
>
> and if I had a 5-level guest, it would have LA57 already set too.
>
> So I think you mean "When paging is enabled" as dhansen correctly points
> out.
>

Ack.

> > and so they can simply be preserved rather than reason about whether
> > or not they need to be set. CR4.PSE has no effect in long mode so it
> > can be omitted.
>
> f4c5ca985012 ("x86_64: Show CR4.PSE on auxiliaries like on BSP")
>
> Please don't forget about git history before doing changes here.
>

My bad - I misunderstood what is going on here.

> > CR4.PGE is used to flush the TLBs, by clearing it if it was set, and
>
> ... to flush TLB entries with the global bit set.
>
> And just like the above commit says, I think the CR4 settings across all
> CPUs on the machine should be the same. So we want to keep PSE.
>
> Removing the CONFIG_X86_5LEVEL ifdeffery is nice, OTOH.
>

Cheers.