linux-kernel - [PATCH v2 02/17] x86/startup_64: Simplify calculation of initial page table address

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Date: Thu, 25 Jan 2024 12:28:21 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: 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>, Borislav Petkov <bp@...en8.de>, 
	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>, 
	Brian Gerst <brgerst@...il.com>, linux-arch@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH v2 02/17] x86/startup_64: Simplify calculation of initial page
 table address

From: Ard Biesheuvel <ardb@...nel.org>

Determining the address of the initial page table to program into CR3
involves:
- taking the physical address
- adding the SME encryption mask

On the primary entry path, the code is mapped using a 1:1 virtual to
physical translation, so the physical address can be taken directly
using a RIP-relative LEA instruction.

On the secondary entry path, the address can be obtained by taking the
offset from the virtual kernel base (__START_kernel_map) and adding the
physical kernel base.

This is all very straight-forward, but the current code makes a mess of
this. Clean this up.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/kernel/head_64.S | 25 ++++++--------------
 1 file changed, 7 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 4017a49d7b76..6d24c2014759 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -113,13 +113,11 @@ SYM_CODE_START_NOALIGN(startup_64)
 	call	__startup_64
 
 	/* Form the CR3 value being sure to include the CR3 modifier */
-	addq	$(early_top_pgt - __START_KERNEL_map), %rax
+	leaq	early_top_pgt(%rip), %rcx
+	addq	%rcx, %rax
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	mov	%rax, %rdi
-	mov	%rax, %r14
-
-	addq	phys_base(%rip), %rdi
 
 	/*
 	 * For SEV guests: Verify that the C-bit is correct. A malicious
@@ -128,12 +126,6 @@ SYM_CODE_START_NOALIGN(startup_64)
 	 * the next RET instruction.
 	 */
 	call	sev_verify_cbit
-
-	/*
-	 * Restore CR3 value without the phys_base which will be added
-	 * below, before writing %cr3.
-	 */
-	 mov	%r14, %rax
 #endif
 
 	jmp 1f
@@ -173,18 +165,18 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	/* Clear %R15 which holds the boot_params pointer on the boot CPU */
 	xorq	%r15, %r15
 
+	/* Derive the runtime physical address of init_top_pgt[] */
+	movq	phys_base(%rip), %rax
+	addq	$(init_top_pgt - __START_KERNEL_map), %rax
+
 	/*
 	 * Retrieve the modifier (SME encryption mask if SME is active) to be
 	 * added to the initial pgdir entry that will be programmed into CR3.
 	 */
 #ifdef CONFIG_AMD_MEM_ENCRYPT
-	movq	sme_me_mask, %rax
-#else
-	xorq	%rax, %rax
+	addq	sme_me_mask(%rip), %rax
 #endif
 
-	/* Form the CR3 value being sure to include the CR3 modifier */
-	addq	$(init_top_pgt - __START_KERNEL_map), %rax
 1:
 
 #ifdef CONFIG_X86_MCE
@@ -211,9 +203,6 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 #endif
 	movq	%rcx, %cr4
 
-	/* Setup early boot stage 4-/5-level pagetables. */
-	addq	phys_base(%rip), %rax
-
 	/*
 	 * Switch to new page-table
 	 *
-- 
2.43.0.429.g432eaa2c6b-goog