[PATCH v6 01/21] x86/head_64: Store boot_params pointer in callee save register

[Ard Biesheuvel <ardb@kernel.org>]

Instead of pushing/popping %RSI to/from the stack every time a function
is called from startup_64(), store it in callee preserved register R15
and grab it from there when its value is actually needed.

Secondary boot does not and should not care about the value of %RSI at
entry, and all the C handlers except the primary boot C entrypoint all
ignore the function arguments entirely. Avoid inadvertent surprises by
zeroing R15 on the secondary boot path explicitly.
 
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/kernel/head_64.S | 23 +++++---------------
 1 file changed, 5 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index c5b9289837dcbad2..dbc80fe098f86860 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -62,6 +62,7 @@ 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
@@ -75,9 +76,7 @@ SYM_CODE_START_NOALIGN(startup_64)
 	shrq	$32,  %rdx
 	wrmsr
 
-	pushq	%rsi
 	call	startup_64_setup_env
-	popq	%rsi
 
 	/* Now switch to __KERNEL_CS so IRET works reliably */
 	pushq	$__KERNEL_CS
@@ -95,10 +94,8 @@ 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		/* Pass boot_params pointer */
 	call	sme_enable
-	popq	%rsi
 #endif
 
 	/* Sanitize CPU configuration */
@@ -111,9 +108,7 @@ 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
@@ -127,8 +122,6 @@ 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).
 	 *
@@ -153,6 +146,8 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	UNWIND_HINT_END_OF_STACK
 	ANNOTATE_NOENDBR
 
+	xorq	%r15, %r15		/* Clear boot_params pointer */
+
 	/*
 	 * Retrieve the modifier (SME encryption mask if SME is active) to be
 	 * added to the initial pgdir entry that will be programmed into CR3.
@@ -199,13 +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
@@ -365,9 +356,7 @@ 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
@@ -403,9 +392,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	pushq $0
 	popfq
 
-	/* rsi is pointer to real mode structure with interesting info.
-	   pass it to C */
-	movq	%rsi, %rdi
+	movq	%r15, %rdi		/* Pass boot_params pointer */
 
 .Ljump_to_C_code:
 	/*
-- 
2.39.2