linux-kernel - [PATCH 1/6] x86/smpboot: Use CPU number instead of APIC ID for single CPU startup

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Date:   Wed, 22 Feb 2023 17:12:56 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Usama Arif <usama.arif@...edance.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Brian Gerst <brgerst@...il.com>
Subject: [PATCH 1/6] x86/smpboot: Use CPU number instead of APIC ID for single CPU startup

Encoding the CPU number directly in smpboot_control skips the APIC ID
lookup when booting a single CPU.  This will enable the boot CPU to use
the same code as secondaries, since the APIC ID array is not populated
during early boot.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/kernel/head_64.S | 12 +++++++-----
 arch/x86/kernel/smpboot.c |  2 +-
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 0e4e53d231db..c1253aa737ca 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -252,20 +252,22 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	jz	.Lsetup_cpu
 
 	/*
-	 * Secondary CPUs find out the offsets via the APIC ID. For parallel
-	 * boot the APIC ID is retrieved from CPUID, otherwise it's encoded
-	 * in smpboot_control:
+	 * 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	APIC ID if STARTUP_APICID_CPUID_xx flags are not set
+	 * Bit 0-24	CPU# if STARTUP_APICID_CPUID_xx flags are not set
 	 */
 	testl	$STARTUP_APICID_CPUID_0B, %edx
 	jnz	.Luse_cpuid_0b
 	testl	$STARTUP_APICID_CPUID_01, %edx
 	jnz	.Luse_cpuid_01
 	andl	$0x0FFFFFFF, %edx
-	jmp	.Lsetup_AP
+	movl	%edx, %ecx
+	jmp	.Linit_cpu_data
 
 .Luse_cpuid_01:
 	mov	$0x01, %eax
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 5b6d72b3d14b..e1a2843c2841 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 | apicid;
+		smpboot_control = STARTUP_SECONDARY | cpu;
 	}
 
 	/* Enable the espfix hack for this CPU */
-- 
2.39.2