[Xen-changelog] [xen stable-4.12] x86/smp: reset x2apic_enabled in smp_send_stop()

[patchbot@xxxxxxx]

commit 3d89e049311669d5a8d826e9280ffdf88e77395c
Author:     David Woodhouse <dwmw@xxxxxxxxxxxx>
AuthorDate: Thu Mar 5 11:21:47 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 5 11:21:47 2020 +0100

    x86/smp: reset x2apic_enabled in smp_send_stop()
    
    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>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: 8b1002ab037aeacdece7723c07ab35ca16c1e22e
    master date: 2020-02-14 18:01:52 +0000
---
 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 cf12ba85b2..6e8a81a676 100644
--- a/xen/arch/x86/smp.c
+++ b/xen/arch/x86/smp.c
@@ -325,6 +325,7 @@ void smp_send_stop(void)
         disable_IO_APIC();
         hpet_disable();
         __stop_this_cpu();
+        x2apic_enabled = (current_local_apic_mode() == APIC_MODE_X2APIC);
         local_irq_enable();
     }
 }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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