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Re: [Xen-devel] [PATCH] x86/xen: Disable APIC PM for Xen PV guests

On 03/04/2014 05:25 AM, David Vrabel wrote:
On 04/03/14 02:40, Boris Ostrovsky wrote:
Xen PV guests support only few APIC registers and writes to
unsupported registers result in WARN_ONs. Most APIC accesses in these
guests  have been eliminated; however, lapic_suspend/resume are still
called (on 32-bit kernels).

We can disable APIC power management in xen_smp_prepare_boot_cpu()
(which is called after APIC has been initialized).
Having looked at another APIC related problem recently, I wonder if Xen
would be better served by having its own APIC driver instead of trying
to piggy-back of some hardware one which doesn't do what we want.

Possibly, although I don't think it will help with this particular issue since this patch is trying to address a problem in non-privileged guests. Such guests don't have ACPI enabled and so no APIC probing is done for them.



Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
  arch/x86/include/asm/apic.h |    2 ++
  arch/x86/kernel/apic/apic.c |    6 ++++++
  arch/x86/xen/smp.c          |    4 ++++
  3 files changed, 12 insertions(+)

An alternative could be to add another op to stuct apic. The advantage would be
the fact that we never have a window of time when APIC PM is enabled. The 
is having yet another op (of which there are already plenty).

We could also avoid loading lapic_syscore_ops but I couldn't think of a good way
to figure out when not to do it.

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 1d2091a..1f100f2 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -687,6 +687,8 @@ extern int default_cpu_present_to_apicid(int mps_cpu);
  extern int default_check_phys_apicid_present(int phys_apicid);
+extern void apic_pm_deactivate(void);
  #endif /* CONFIG_X86_LOCAL_APIC */
  extern void irq_enter(void);
  extern void irq_exit(void);
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 7f26c9a..60607b3 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2424,6 +2424,11 @@ static void apic_pm_activate(void)
        apic_pm_state.active = 1;
+void apic_pm_deactivate(void)
+       apic_pm_state.active = 0;
  static int __init init_lapic_sysfs(void)
        /* XXX: remove suspend/resume procs if !apic_pm_state.active? */
@@ -2439,6 +2444,7 @@ core_initcall(init_lapic_sysfs);
  #else /* CONFIG_PM */
static void apic_pm_activate(void) { }
+void apic_pm_deactivate(void) { }
#endif /* CONFIG_PM */ diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index a18eadd..33115d8 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -298,6 +298,10 @@ static void __init xen_smp_prepare_boot_cpu(void)
+               apic_pm_deactivate();
         * The alternative logic (which patches the unlock/lock) runs before

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