linux-kernel - [PATCH v4 18/21] x86/head_64: Store boot_params pointer in callee-preserved register

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Message-Id: <20230602101313.3557775-19-ardb@kernel.org>
Date:   Fri,  2 Jun 2023 12:13:10 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-efi@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Evgeniy Baskov <baskov@...ras.ru>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>,
        Kees Cook <keescook@...omium.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>
Subject: [PATCH v4 18/21] x86/head_64: Store boot_params pointer in callee-preserved register

Instead of pushing/popping %RSI to/from the stack every time a function
is called from startup_64(), store it in a callee preserved register
and grab it from there when its value is actualled needed.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/kernel/head_64.S | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index a5df3e994f04f10f..95b12fdae10e1dc9 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -60,6 +60,7 @@ SYM_CODE_START_NOALIGN(startup_64)
 	 * compiled to run at we first fixup the physical addresses in our page
 	 * tables and then reload them.
 	 */
+	mov	%rsi, %r15		/* Preserve boot_params pointer */
 
 	/* Set up the stack for verify_cpu() */
 	leaq	(__end_init_task - PTREGS_SIZE)(%rip), %rsp
@@ -73,9 +74,7 @@ SYM_CODE_START_NOALIGN(startup_64)
 	shrq	$32,  %rdx
 	wrmsr
 
-	pushq	%rsi
 	call	startup_64_setup_env
-	popq	%rsi
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	/*
@@ -84,10 +83,8 @@ SYM_CODE_START_NOALIGN(startup_64)
 	 * which needs to be done before any CPUID instructions are executed in
 	 * subsequent code.
 	 */
-	movq	%rsi, %rdi
-	pushq	%rsi
+	movq	%r15, %rdi
 	call	sme_enable
-	popq	%rsi
 #endif
 
 	/* Now switch to __KERNEL_CS so IRET works reliably */
@@ -109,9 +106,7 @@ SYM_CODE_START_NOALIGN(startup_64)
 	 * programmed into CR3.
 	 */
 	leaq	_text(%rip), %rdi
-	pushq	%rsi
 	call	__startup_64
-	popq	%rsi
 
 	/* Form the CR3 value being sure to include the CR3 modifier */
 	addq	$(early_top_pgt - __START_KERNEL_map), %rax
@@ -200,10 +195,8 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	 * %rsi carries pointer to realmode data and is callee-clobbered. Save
 	 * and restore it.
 	 */
-	pushq	%rsi
 	movq	%rax, %rdi
 	call	sev_verify_cbit
-	popq	%rsi
 
 	/*
 	 * Switch to new page-table
@@ -294,9 +287,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	wrmsr
 
 	/* Setup and Load IDT */
-	pushq	%rsi
 	call	early_setup_idt
-	popq	%rsi
 
 	/* Check if nx is implemented */
 	movl	$0x80000001, %eax
@@ -334,7 +325,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
 	/* rsi is pointer to real mode structure with interesting info.
 	   pass it to C */
-	movq	%rsi, %rdi
+	movq	%r15, %rdi
 
 .Ljump_to_C_code:
 	/*
-- 
2.39.2