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

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Date:	Tue, 04 Mar 2014 16:17:22 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	konrad.wilk@...cle.com, xen-devel@...ts.xenproject.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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.

-boris

>
> David
>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>> ---
>>   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 downside
>> 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);
>>   #endif
>>   
>> +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)
>>   
>>   		xen_filter_cpu_maps();
>>   		xen_setup_vcpu_info_placement();
>> +
>> +#ifdef CONFIG_X86_LOCAL_APIC
>> +		apic_pm_deactivate();
>> +#endif
>>   	}
>>   	/*
>>   	 * The alternative logic (which patches the unlock/lock) runs before

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