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

From: Sander Eikelenboom
Date: Wed Dec 02 2015 - 12:35:23 EST


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@xxxxxxxxxxxxxx>

--
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@xxxxxxxxxx>
---
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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