[tip: objtool/core] x86/xen: don't let xen_pv_play_dead() return

[tip-bot2 for Juergen Gross]

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     076cbf5d216377087e135a0ae81844f7835cdc15
Gitweb:        https://git.kernel.org/tip/076cbf5d216377087e135a0ae81844f7835cdc15
Author:        Juergen Gross <jgross@xxxxxxxx>
AuthorDate:    Fri, 25 Nov 2022 07:32:47 +01:00
Committer:     Juergen Gross <jgross@xxxxxxxx>
CommitterDate: Tue, 17 Jan 2023 09:05:28 +01:00

x86/xen: don't let xen_pv_play_dead() return

A function called via the paravirt play_dead() hook should not return
to the caller.

xen_pv_play_dead() has a problem in this regard, as it currently will
return in case an offlined cpu is brought to life again. This can be
changed only by doing basically a longjmp() to cpu_bringup_and_idle(),
as the hypercall for bringing down the cpu will just return when the
cpu is coming up again. Just re-initializing the cpu isn't possible,
as the Xen hypervisor will deny that operation.

So introduce xen_cpu_bringup_again() resetting the stack and calling
cpu_bringup_and_idle(), which can be called after HYPERVISOR_vcpu_op()
in xen_pv_play_dead().

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20221125063248.30256-2-jgross@xxxxxxxx
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/xen/smp.h      |  2 ++
 arch/x86/xen/smp_pv.c   | 13 ++-----------
 arch/x86/xen/xen-head.S |  7 +++++++
 3 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/arch/x86/xen/smp.h b/arch/x86/xen/smp.h
index bd02f9d..6e90a31 100644
--- a/arch/x86/xen/smp.h
+++ b/arch/x86/xen/smp.h
@@ -21,6 +21,8 @@ void xen_smp_send_reschedule(int cpu);
 void xen_smp_send_call_function_ipi(const struct cpumask *mask);
 void xen_smp_send_call_function_single_ipi(int cpu);
 
+void xen_cpu_bringup_again(unsigned long stack);
+
 struct xen_common_irq {
 	int irq;
 	char *name;
diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 6175f2c..a97b050 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -385,17 +385,8 @@ static void xen_pv_play_dead(void) /* used only with HOTPLUG_CPU */
 {
 	play_dead_common();
 	HYPERVISOR_vcpu_op(VCPUOP_down, xen_vcpu_nr(smp_processor_id()), NULL);
-	cpu_bringup();
-	/*
-	 * commit 4b0c0f294 (tick: Cleanup NOHZ per cpu data on cpu down)
-	 * clears certain data that the cpu_idle loop (which called us
-	 * and that we return from) expects. The only way to get that
-	 * data back is to call:
-	 */
-	tick_nohz_idle_enter();
-	tick_nohz_idle_stop_tick_protected();
-
-	cpuhp_online_idle(CPUHP_AP_ONLINE_IDLE);
+	xen_cpu_bringup_again((unsigned long)task_pt_regs(current));
+	BUG();
 }
 
 #else /* !CONFIG_HOTPLUG_CPU */
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index ffaa621..e36ea42 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -76,6 +76,13 @@ SYM_CODE_START(asm_cpu_bringup_and_idle)
 
 	call cpu_bringup_and_idle
 SYM_CODE_END(asm_cpu_bringup_and_idle)
+
+SYM_CODE_START(xen_cpu_bringup_again)
+	UNWIND_HINT_FUNC
+	mov	%rdi, %rsp
+	UNWIND_HINT_REGS
+	call	cpu_bringup_and_idle
+SYM_CODE_END(xen_cpu_bringup_again)
 .popsection
 #endif
 #endif