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From: Ard Biesheuvel <ardb@kernel.org>
Date: Thu, 29 Feb 2024 23:36:36 +0100
Message-ID: <CAMj1kXGigJnbCiXC0PjnWX-2R3y+KPeQLKHETe8DCEhiX2wYnQ@mail.gmail.com>
Subject: Re: [PATCH v7 1/9] x86/startup_64: Simplify CR4 handling in startup code
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>, Brian Gerst <brgerst@gmail.com>
Content-Type: text/plain; charset="UTF-8"

On Wed, 28 Feb 2024 at 14:45, Borislav Petkov <bp@alien8.de> wrote:
>
> On Tue, Feb 27, 2024 at 04:19:09PM +0100, Ard Biesheuvel wrote:
> > +     /*
> > +      * Create a mask of CR4 bits to preserve. Omit PGE in order to clean
> > +      * global 1:1 translations from the TLBs.
>
> Brian raised this question when exactly global entries get flushed and
> I was looking for the exact definition in the SDM, here's what I'll do
> ontop:
>
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 2d8762887c6a..24df91535062 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -186,8 +186,13 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
>  1:
>
>         /*
> -        * Create a mask of CR4 bits to preserve. Omit PGE in order to clean
> +        * Create a mask of CR4 bits to preserve. Omit PGE in order to flush
>          * global 1:1 translations from the TLBs.
> +        *
> +        * From the SDM:
> +        * "If CR4.PGE is changing from 0 to 1, there were no global TLB
> +        *  entries before the execution; if CR4.PGE is changing from 1 to 0,
> +        *  there will be no global TLB entries after the execution."
>          */
>         movl    $(X86_CR4_PAE | X86_CR4_LA57), %edx
>  #ifdef CONFIG_X86_MCE
> ---
>
> And how it is perfectly clear.
>

Looks good to me - thanks.