Re: [Xen-devel] linux 4.4 Regression: 100% cpu usage on idle pv guest under Xen with single vcpu.

[David Vrabel <david.vrabel@citrix.com>]

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
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");
-- 
2.1.4