linux-kernel - [PATCH] x86/boot: Clean up handling of boot

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Date:   Fri, 31 Mar 2023 14:28:39 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Brian Gerst <brgerst@...il.com>
Subject: [PATCH] x86/boot: Clean up handling of boot_params pointer

On entry from the bootloader, RSI contains the pointer to the
boot_params data structure.  Since the RSI register can be clobbered
when calling C functions, it is saved and restored around every call.
Instead, move it to the R12 register, which is preserved across calls.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/kernel/head_64.S | 29 ++++++++++-------------------
 1 file changed, 10 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index a5df3e994f04..0d130ca2e0a3 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -49,8 +49,6 @@ SYM_CODE_START_NOALIGN(startup_64)
 	 * for us.  These identity mapped page tables map all of the
 	 * kernel pages and possibly all of memory.
 	 *
-	 * %rsi holds a physical pointer to real_mode_data.
-	 *
 	 * We come here either directly from a 64bit bootloader, or from
 	 * arch/x86/boot/compressed/head_64.S.
 	 *
@@ -61,6 +59,12 @@ SYM_CODE_START_NOALIGN(startup_64)
 	 * tables and then reload them.
 	 */
 
+	/*
+	 * RSI holds a physical pointer to real_mode_data.  Move it to R12,
+	 * which is preserved across C function calls.
+	 */
+	movq	%rsi, %r12
+
 	/* Set up the stack for verify_cpu() */
 	leaq	(__end_init_task - PTREGS_SIZE)(%rip), %rsp
 
@@ -73,9 +77,7 @@ 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 +86,8 @@ 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	%r12, %rdi
 	call	sme_enable
-	popq	%rsi
 #endif
 
 	/* Now switch to __KERNEL_CS so IRET works reliably */
@@ -109,9 +109,8 @@ SYM_CODE_START_NOALIGN(startup_64)
 	 * programmed into CR3.
 	 */
 	leaq	_text(%rip), %rdi
-	pushq	%rsi
+	movq	%r12, %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
@@ -125,8 +124,6 @@ SYM_CODE_START(secondary_startup_64)
 	 * At this point the CPU runs in 64bit mode CS.L = 1 CS.D = 0,
 	 * and someone has loaded a mapped page table.
 	 *
-	 * %rsi holds a physical pointer to real_mode_data.
-	 *
 	 * We come here either from startup_64 (using physical addresses)
 	 * or from trampoline.S (using virtual addresses).
 	 *
@@ -197,13 +194,9 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	 * hypervisor could lie about the C-bit position to perform a ROP
 	 * attack on the guest by writing to the unencrypted stack and wait for
 	 * the next RET instruction.
-	 * %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
@@ -332,9 +323,9 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	pushq $0
 	popfq
 
-	/* rsi is pointer to real mode structure with interesting info.
+	/* R12 is pointer to real mode structure with interesting info.
 	   pass it to C */
-	movq	%rsi, %rdi
+	movq	%r12, %rdi
 
 .Ljump_to_C_code:
 	/*
-- 
2.39.2