linux-kernel - [PATCH 2/6] x86/smpboot: Use current

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Date:   Wed, 22 Feb 2023 17:12:57 -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 2/6] x86/smpboot: Use current_task to get idle thread

The idle_threads array is not populated during early boot.  Use
current_task instead, which is initialized to init_task for the boot
CPU.

Also simplify start_cpu0().  Since the boot CPU never really goes
offline, the GSBASE is still set up and can be used for per-cpu
accesses.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/kernel/asm-offsets.c | 1 +
 arch/x86/kernel/head_64.S     | 7 ++-----
 kernel/smpboot.c              | 2 +-
 3 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index 8650f29387e0..445bce086717 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -114,6 +114,7 @@ static void __used common(void)
 	OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);
 	OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
 	OFFSET(X86_top_of_stack, pcpu_hot, top_of_stack);
+	OFFSET(X86_current_task, pcpu_hot, current_task);
 #ifdef CONFIG_CALL_DEPTH_TRACKING
 	OFFSET(X86_call_depth, pcpu_hot, call_depth);
 #endif
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index c1253aa737ca..c32e5b06a9ce 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -315,7 +315,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	movq	%rcx, early_gdt_descr_base(%rip)
 
 	/* Find the idle task stack */
-	movq	idle_threads(%rbx), %rcx
+	movq	pcpu_hot + X86_current_task(%rbx), %rcx
 	movq	TASK_threadsp(%rcx), %rcx
 	movq	%rcx, initial_stack(%rip)
 #endif /* CONFIG_SMP */
@@ -460,12 +460,9 @@ SYM_CODE_END(secondary_startup_64)
 SYM_CODE_START(start_cpu0)
 	ANNOTATE_NOENDBR
 	UNWIND_HINT_EMPTY
-	/* Load the per-cpu base for CPU#0 */
-	leaq	__per_cpu_offset(%rip), %rbx
-	movq	(%rbx), %rbx
 
 	/* Find the idle task stack */
-	movq	idle_threads(%rbx), %rcx
+	movq	PER_CPU_VAR(pcpu_hot) + X86_current_task, %rcx
 	movq	TASK_threadsp(%rcx), %rsp
 
 	jmp	.Ljump_to_C_code
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index a18a21dff9bc..2c7396da470c 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -25,7 +25,7 @@
  * For the hotplug case we keep the task structs around and reuse
  * them.
  */
-DEFINE_PER_CPU(struct task_struct *, idle_threads);
+static DEFINE_PER_CPU(struct task_struct *, idle_threads);
 
 struct task_struct *idle_thread_get(unsigned int cpu)
 {
-- 
2.39.2