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Date: Wed, 28 Feb 2024 14:45:00 +0100
From: Borislav Petkov <bp@alien8.de>
To: Ard Biesheuvel <ardb+git@google.com>
Cc: linux-kernel@vger.kernel.org, Ard Biesheuvel <ardb@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>
Subject: Re: [PATCH v7 1/9] x86/startup_64: Simplify CR4 handling in startup
 code
Message-ID: <20240228134500.GHZd843I43EccJD1Ak@fat_crate.local>
References: <20240227151907.387873-11-ardb+git@google.com>
 <20240227151907.387873-12-ardb+git@google.com>
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In-Reply-To: <20240227151907.387873-12-ardb+git@google.com>

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