[PATCH 6/6] x86/smpboot: Simplify boot CPU setup

Brian Gerst
2023-02-22T22:14:02Z

Now that the per-cpu GSBASE, stack, and GDT descriptor can be derived
dynammically 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>
---
arch/x86/include/asm/smp.h | 1 -
arch/x86/kernel/head_64.S | 25 +++++++------------------
arch/x86/kernel/smpboot.c | 6 +++---
3 files changed, 10 insertions(+), 22 deletions(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index b4b29e052b6e..32c9157238c0 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -202,7 +202,6 @@ 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

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 e1a2843c2841..c159a5c2df9f 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) {
--
2.39.2