linux-kernel - Re: [PATCH v7 1/9] x86/startup_64: Simplify CR4 handling in startup code

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Date: Wed, 28 Feb 2024 14:45:00 +0100
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.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 1/9] x86/startup_64: Simplify CR4 handling in startup
 code

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.

Thx.

-- 
Regards/Gruss,
    Boris.

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