[PATCH v11 12/12] x86/smpboot: Simplify boot CPU setup

[Usama Arif <usama.arif@bytedance.com>]

From: Brian Gerst <brgerst@gmail.com>

Now that the per-cpu GSBASE, stack, and GDT descriptor can be derived
dynamically by CPU number, the boot CPU can use a fixed CPU number and
take the same path as secondary CPUs.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Tested-by: Usama Arif <usama.arif@bytedance.com>
Signed-off-by: Usama Arif <usama.arif@bytedance.com>
---
 arch/x86/include/asm/smp.h |  5 ++---
 arch/x86/kernel/head_64.S  | 25 +++++++------------------
 arch/x86/kernel/smpboot.c  |  6 +++---
 3 files changed, 12 insertions(+), 24 deletions(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index b4b29e052b6e..97a36d029b0e 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -202,8 +202,7 @@ extern unsigned int smpboot_control;
 #endif /* !__ASSEMBLY__ */
 
 /* Control bits for startup_64 */
-#define STARTUP_SECONDARY	0x80000000
-#define STARTUP_APICID_CPUID_0B	0x40000000
-#define STARTUP_APICID_CPUID_01	0x20000000
+#define STARTUP_APICID_CPUID_0B	0x80000000
+#define STARTUP_APICID_CPUID_01	0x40000000
 
 #endif /* _ASM_X86_SMP_H */
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 9ed87ba0609f..949c13b26811 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -235,28 +235,22 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR // above
 
 #ifdef CONFIG_SMP
-	/* Is this the boot CPU coming up? */
-	movl	smpboot_control(%rip), %edx
-	testl	$STARTUP_SECONDARY, %edx
-	jz	.Linit_cpu0_data
-
 	/*
 	 * For parallel boot, the APIC ID is retrieved from CPUID, and then
 	 * used to look up the CPU number.  For booting a single CPU, the
 	 * CPU number is encoded in smpboot_control.
 	 *
-	 * Bit 31	STARTUP_SECONDARY flag (checked above)
 	 * Bit 30	STARTUP_APICID_CPUID_0B flag (use CPUID 0x0b)
 	 * Bit 29	STARTUP_APICID_CPUID_01 flag (use CPUID 0x01)
 	 * Bit 0-24	CPU# if STARTUP_APICID_CPUID_xx flags are not set
 	 */
-	testl	$STARTUP_APICID_CPUID_0B, %edx
+	movl	smpboot_control(%rip), %ecx
+	testl	$STARTUP_APICID_CPUID_0B, %ecx
 	jnz	.Luse_cpuid_0b
-	testl	$STARTUP_APICID_CPUID_01, %edx
+	testl	$STARTUP_APICID_CPUID_01, %ecx
 	jnz	.Luse_cpuid_01
-	andl	$0x0FFFFFFF, %edx
-	movl	%edx, %ecx
-	jmp	.Linit_cpu_data
+	andl	$0x0FFFFFFF, %ecx
+	jmp	.Lsetup_cpu
 
 .Luse_cpuid_01:
 	mov	$0x01, %eax
@@ -277,7 +271,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
 .Lfind_cpunr:
 	cmpl	(%rbx,%rcx,4), %edx
-	jz	.Linit_cpu_data
+	jz	.Lsetup_cpu
 	inc	%ecx
 	cmpl	nr_cpu_ids(%rip), %ecx
 	jb	.Lfind_cpunr
@@ -291,18 +285,13 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	hlt
 	jmp	1b
 
-.Linit_cpu0_data:
-	movq	__per_cpu_offset(%rip), %rdx
-	jmp	.Lsetup_cpu
-
-.Linit_cpu_data:
+.Lsetup_cpu:
 	/* Get the per cpu offset for the given CPU# which is in ECX */
 	movq	__per_cpu_offset(,%rcx,8), %rdx
 #else
 	xorl	%edx, %edx
 #endif /* CONFIG_SMP */
 
-.Lsetup_cpu:
 	/*
 	 * Setup a boot time stack - Any secondary CPU will have lost its stack
 	 * by now because the cr3-switch above unmaps the real-mode stack
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 69ef0860feea..9d956571ecc1 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1140,7 +1140,7 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle,
 		early_gdt_descr.address = (unsigned long)get_cpu_gdt_rw(cpu);
 		initial_stack  = idle->thread.sp;
 	} else if (!do_parallel_bringup) {
-		smpboot_control = STARTUP_SECONDARY | cpu;
+		smpboot_control = cpu;
 	}
 
 	/* Enable the espfix hack for this CPU */
@@ -1580,7 +1580,7 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
 		 */
 		if (eax) {
 			pr_debug("Using CPUID 0xb for parallel CPU startup\n");
-			smpboot_control = STARTUP_SECONDARY | STARTUP_APICID_CPUID_0B;
+			smpboot_control = STARTUP_APICID_CPUID_0B;
 		} else {
 			pr_info("Disabling parallel bringup because CPUID 0xb looks untrustworthy\n");
 			do_parallel_bringup = false;
@@ -1588,7 +1588,7 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
 	} else if (do_parallel_bringup) {
 		/* Without X2APIC, what's in CPUID 0x01 should suffice. */
 		pr_debug("Using CPUID 0x1 for parallel CPU startup\n");
-		smpboot_control = STARTUP_SECONDARY | STARTUP_APICID_CPUID_01;
+		smpboot_control = STARTUP_APICID_CPUID_01;
 	}
 
 	if (do_parallel_bringup) {
-- 
2.25.1