[PATCH v2 01/17] x86/startup_64: Drop long return to initial_code pointer

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

Since commit 866b556efa12 ("x86/head/64: Install startup GDT"), the
primary startup sequence sets the code segment register (CS) to __KERNEL_CS
before calling into the startup code shared between primary and
secondary boot.

This means a simple indirect call is sufficient here.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/kernel/head_64.S | 35 ++------------------
 1 file changed, 3 insertions(+), 32 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index d4918d03efb4..4017a49d7b76 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -428,39 +428,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	movq	%r15, %rdi
 
 .Ljump_to_C_code:
-	/*
-	 * Jump to run C code and to be on a real kernel address.
-	 * Since we are running on identity-mapped space we have to jump
-	 * to the full 64bit address, this is only possible as indirect
-	 * jump.  In addition we need to ensure %cs is set so we make this
-	 * a far return.
-	 *
-	 * Note: do not change to far jump indirect with 64bit offset.
-	 *
-	 * AMD does not support far jump indirect with 64bit offset.
-	 * AMD64 Architecture Programmer's Manual, Volume 3: states only
-	 *	JMP FAR mem16:16 FF /5 Far jump indirect,
-	 *		with the target specified by a far pointer in memory.
-	 *	JMP FAR mem16:32 FF /5 Far jump indirect,
-	 *		with the target specified by a far pointer in memory.
-	 *
-	 * Intel64 does support 64bit offset.
-	 * Software Developer Manual Vol 2: states:
-	 *	FF /5 JMP m16:16 Jump far, absolute indirect,
-	 *		address given in m16:16
-	 *	FF /5 JMP m16:32 Jump far, absolute indirect,
-	 *		address given in m16:32.
-	 *	REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
-	 *		address given in m16:64.
-	 */
-	pushq	$.Lafter_lret	# put return address on stack for unwinder
 	xorl	%ebp, %ebp	# clear frame pointer
-	movq	initial_code(%rip), %rax
-	pushq	$__KERNEL_CS	# set correct cs
-	pushq	%rax		# target address in negative space
-	lretq
-.Lafter_lret:
-	ANNOTATE_NOENDBR
+	ANNOTATE_RETPOLINE_SAFE
+	callq	*initial_code(%rip)
+	int3
 SYM_CODE_END(secondary_startup_64)
 
 #include "verify_cpu.S"
-- 
2.43.0.429.g432eaa2c6b-goog