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[Xen-devel] [PATCH 1/8] x86/smp: reset x2apic_enabled in smp_send_stop()

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Just before smp_send_stop() re-enables interrupts when shutting down
for reboot or kexec, it calls __stop_this_cpu() which in turn calls
disable_local_APIC(), which puts the APIC back in to the mode Xen found
it in at boot.

If that means turning x2APIC off and going back into xAPIC mode, then
a timer interrupt occurring just after interrupts come back on will
lead to a GP# when apic_timer_interrupt() attempts to ack the IRQ
through the EOI register in x2APIC MSR 0x80b:

(XEN) Executing kexec image on cpu0
(XEN) ----[ Xen-4.14-unstable  x86_64  debug=n   Not tainted ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff82d08026c139>] apic_timer_interrupt+0x29/0x40
(XEN) RFLAGS: 0000000000010046   CONTEXT: hypervisor
(XEN) rax: 0000000000000000   rbx: 00000000000000fa   rcx: 000000000000080b
(XEN) Xen code around <ffff82d08026c139> (apic_timer_interrupt+0x29/0x40):
(XEN)  c0 b9 0b 08 00 00 89 c2 <0f> 30 31 ff e9 0e c9 fb ff 0f 1f 40 00 66 2e 0f
(XEN) Xen call trace:
(XEN)    [<ffff82d08026c139>] R apic_timer_interrupt+0x29/0x40
(XEN)    [<ffff82d080283825>] S do_IRQ+0x95/0x750
(XEN)    [<ffff82d0802a0ad2>] S smp_send_stop+0x42/0xd0

We can't clear the global x2apic_enabled variable in disable_local_APIC()
itself because that runs on each CPU. Instead, correct it (by using
current_local_apic_mode()) in smp_send_stop() while interrupts are still
disabled immediately after calling __stop_this_cpu() for the boot CPU,
after all other CPUs have been stopped.

cf: d639bdd9bbe ("x86/apic: Disable the LAPIC later in smp_send_stop()")
    ... which didn't quite fix it completely.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
 xen/arch/x86/smp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
index 65eb7cbda8..fac295fa6f 100644
--- a/xen/arch/x86/smp.c
+++ b/xen/arch/x86/smp.c
@@ -354,6 +354,7 @@ void smp_send_stop(void)
+        x2apic_enabled = (current_local_apic_mode() == APIC_MODE_X2APIC);

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