Patch "x86/paravirt: Prevent rtc_cmos platform device init on PV guests" has been added to the 4.3-stable tree

This is a note to let you know that I've just added the patch titled

x86/paravirt: Prevent rtc_cmos platform device init on PV guests

to the 4.3-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-paravirt-prevent-rtc_cmos-platform-device-init-on-pv-guests.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Vrabel <[email protected]>
Date: Fri, 11 Dec 2015 09:07:53 -0500
Subject: x86/paravirt: Prevent rtc_cmos platform device init on PV guests

From: David Vrabel <[email protected]>

commit d8c98a1d1488747625ad6044d423406e17e99b7a upstream.

Adding the rtc platform device in non-privileged Xen PV guests causes an IRQ conflict because these guests do not have legacy PIC and may allocate irqs in the legacy range.

In a single VCPU Xen 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)

We can avoid this problem by realizing that unprivileged PV guests (both Xen and lguests) are not supposed to have rtc_cmos device and so adding it is not necessary.

Privileged guests (i.e. Xen's dom0) do use it but they should not have irq conflicts since they allocate irqs above legacy range (above gsi_top, in fact).

Instead of explicitly testing whether the guest is privileged we can extend pv_info structure to include information about guest's RTC support.

---
 arch/x86/include/asm/paravirt.h       |    6 ++++++
 arch/x86/include/asm/paravirt_types.h |    5 +++++
 arch/x86/include/asm/processor.h      |    1 +
 arch/x86/kernel/rtc.c                 |    3 +++
 arch/x86/lguest/boot.c                |    1 +
 arch/x86/xen/enlighten.c              |    4 +++-
 6 files changed, 19 insertions(+), 1 deletion(-)

@@ -19,6 +19,12 @@ static inline int paravirt_enabled(void)
 	return pv_info.paravirt_enabled;
 }
 
+static inline int paravirt_has_feature(unsigned int feature)
+{
+	WARN_ON_ONCE(!pv_info.paravirt_enabled);
+	return (pv_info.features & feature);
+}
+
 static inline void load_sp0(struct tss_struct *tss,
 			     struct thread_struct *thread)
 {

@@ -70,9 +70,14 @@ struct pv_info {
 #endif
 
 	int paravirt_enabled;
+	unsigned int features;	  /* valid only if paravirt_enabled is set */
 	const char *name;
 };
 
+#define paravirt_has(x) paravirt_has_feature(PV_SUPPORTED_##x)
+/* Supported features */
+#define PV_SUPPORTED_RTC        (1<<0)
+
 struct pv_init_ops {
 	/*
 	 * Patch may replace one of the defined code sequences with

@@ -472,6 +472,7 @@ static inline unsigned long current_top_
 #else
 #define __cpuid			native_cpuid
 #define paravirt_enabled()	0
+#define paravirt_has(x) 	0
 
 static inline void load_sp0(struct tss_struct *tss,
 			    struct thread_struct *thread)

@@ -200,6 +200,9 @@ static __init int add_rtc_cmos(void)
 	}
 #endif
 
+	if (paravirt_enabled() && !paravirt_has(RTC))
+		return -ENODEV;
+
 	platform_device_register(&rtc_device);
 	dev_info(&rtc_device.dev,
 		 "registered platform RTC device (no PNP device found)\n");

@@ -1414,6 +1414,7 @@ __init void lguest_init(void)
 	pv_info.kernel_rpl = 1;
 	/* Everyone except Xen runs with this set. */
 	pv_info.shared_kernel_pmd = 1;
+	pv_info.features = 0;
 
 	/*
 	 * We set up all the lguest overrides for sensitive operations.  These

@@ -1191,7 +1191,7 @@ static const struct pv_info xen_info __i
 #ifdef CONFIG_X86_64
 	.extra_user_64bit_cs = FLAT_USER_CS64,
 #endif
-
+	.features = 0,
 	.name = "Xen",
 };
 

@@ -1534,6 +1534,8 @@ asmlinkage __visible void __init xen_sta
 
 	/* Install Xen paravirt ops */
 	pv_info = xen_info;
+	if (xen_initial_domain())
+		pv_info.features |= PV_SUPPORTED_RTC;
 	pv_init_ops = xen_init_ops;
 	pv_apic_ops = xen_apic_ops;
 	if (!xen_pvh_domain()) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/x86-xen-don-t-reset-vcpu_info-on-a-cancelled-suspend.patch queue-4.3/x86-paravirt-prevent-rtc_cmos-platform-device-init-on-pv-guests.patch queue-4.3/xen-gntdev-grant-maps-should-not-be-subject-to-numa-balancing.patch