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Date: Wed, 02 Dec 2015 18:30:59 +0100
From: Sander Eikelenboom <linux@eikelenboom.it>
To: David Vrabel <david.vrabel@citrix.com>
Cc: boris.ostrovsky@oracle.com, linux-kernel@vger.kernel.org,
        xen-devel@lists.xen.org
Subject: Re: [Xen-devel] linux 4.4 Regression: 100% cpu usage on idle pv
 guest under Xen with single vcpu.
In-Reply-To: <565F0666.9030602@citrix.com>
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 <565F0666.9030602@citrix.com>
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On 2015-12-02 15:55, David Vrabel wrote:
> On 28/11/15 15:47, Sander Eikelenboom wrote:
>> genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000 
>> (rtc0)
> 
> We shouldn't register an rtc_cmos device because its legacy irq
> conflicts with the irq needed for hvc0.  For a multi VCPU guest irq 8 
> is
> in use for the pv spinlocks and this gets requested first, preventing
> the rtc device from probing.
> 
> Does this patch fix it for you?
> 
> David

It does, thanks.

Reported-and-tested-by: Sander Eikelenboom <linux@eikelenboom.it>

--
Sander

> 8<--------------------
> x86: rtc_cmos platform device requires legacy irqs
> 
> Adding the rtc platform device when there are no legacy irqs (no
> legacy PIC) causes a conflict with other devices that end up using the
> same irq number.
> 
> In a single VCPU PV guest we should have:
> 
> /proc/interrupts:
>            CPU0
>   0:       4934  xen-percpu-virq      timer0
>   1:          0  xen-percpu-ipi       spinlock0
>   2:          0  xen-percpu-ipi       resched0
>   3:          0  xen-percpu-ipi       callfunc0
>   4:          0  xen-percpu-virq      debug0
>   5:          0  xen-percpu-ipi       callfuncsingle0
>   6:          0  xen-percpu-ipi       irqwork0
>   7:        321   xen-dyn-event     xenbus
>   8:         90   xen-dyn-event     hvc_console
>   ...
> 
> But hvc_console cannot get its interrupt because it is already in use
> by rtc0 and the console does not work.
> 
>   genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000 
> (rtc0)
> 
> The rtc_cmos device requires a particular legacy irq so don't add it
> if there are no legacy irqs.
> 
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
>  arch/x86/kernel/rtc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/rtc.c b/arch/x86/kernel/rtc.c
> index cd96852..07c70f1 100644
> --- a/arch/x86/kernel/rtc.c
> +++ b/arch/x86/kernel/rtc.c
> @@ -14,6 +14,7 @@
>  #include <asm/time.h>
>  #include <asm/intel-mid.h>
>  #include <asm/rtc.h>
> +#include <asm/i8259.h>
> 
>  #ifdef CONFIG_X86_32
>  /*
> @@ -200,6 +201,10 @@ static __init int add_rtc_cmos(void)
>  	}
>  #endif
> 
> +	/* RTC uses legacy IRQs. */
> +	if (!nr_legacy_irqs())
> +		return -ENODEV;
> +
>  	platform_device_register(&rtc_device);
>  	dev_info(&rtc_device.dev,
>  		 "registered platform RTC device (no PNP device found)\n");
--
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