linux-kernel - [PATCH v4] x86/hyperv: Handle unknown NMIs on one CPU when unknown_nmi

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Message-Id: <20161202100720.28121-1-vkuznets@redhat.com>
Date:   Fri,  2 Dec 2016 11:07:20 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     x86@...nel.org, devel@...uxdriverproject.org
Cc:     linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH v4] x86/hyperv: Handle unknown NMIs on one CPU when unknown_nmi_panic

There is a feature in Hyper-V ('Debug-VM --InjectNonMaskableInterrupt')
which injects NMI to the guest. We may want to crash the guest and do kdump
on this NMI by enabling unknown_nmi_panic. To make kdump succeed we need to
allow the kdump kernel to re-establish VMBus connection so it will see
VMBus devices (storage, network,..).

To properly unload VMBus making it possible to start over during kdump we
need to do the following:
- Send an 'unload' message to the hypervisor. This can be done on any CPU
  so we do this the crashing CPU.
- Receive the 'unload finished' reply message. WS2012R2 delivers this
  message to the CPU which was used to establish VMBus connection during
  module load and this CPU may differ from the CPU sending 'unload'.

Receiving a VMBus message means the following:
- There is a per-CPU slot in memory for one message. This slot can in
  theory be accessed by any CPU.
- We get an interrupt on the CPU when a message was placed into the slot.
- When we read the message we need to clear the slot and signal the fact
  to the hypervisor. In case there are more messages to this CPU pending
  the hypervisor will deliver the next message. The signaling is done by
  writing to an MSR so this can only be done on the appropriate CPU.

To avoid doing cross-CPU work on crash we have vmbus_wait_for_unload()
function which checks message slots for all CPUs in a loop waiting for the
'unload finished' messages. However, there is an issue which arises when
these conditions are met:
- We're crashing on a CPU which is different from the one which was used
  to initially contact the hypervisor.
- The CPU which was used for the initial contact is blocked with interrupts
  disabled and there is a message pending in the message slot.
In this case we won't be able to read the 'unload finished' message on the
crashing CPU. This is reproducible when we receive unknown NMIs on all CPUs
simultaneously: the first CPU entering panic() will proceed to crash and
all other CPUs will stop themselves with interrupts disabled.

The suggested solution is to handle unknown NMIs for Hyper-V guests on the
first CPU which gets them only. This will allow us to rely on VMBus
interrupt handler being able to receive the 'unload finish' message in
case it is delivered to a different CPU.

The issue is not reproducible on WS2016 as Debug-VM delivers NMI to the
boot CPU only, WS2012R2 and earlier Hyper-V versions are affected.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Acked-by: K. Y. Srinivasan <kys@...rosoft.com>
---
Changes since v3:
- Rewrite the commit message. Hope it's more readable now...
 [Thomas Gleixner]
---
 arch/x86/kernel/cpu/mshyperv.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 8f44c5a..f228f74 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -31,6 +31,7 @@
 #include <asm/apic.h>
 #include <asm/timer.h>
 #include <asm/reboot.h>
+#include <asm/nmi.h>
 
 struct ms_hyperv_info ms_hyperv;
 EXPORT_SYMBOL_GPL(ms_hyperv);
@@ -158,6 +159,26 @@ static unsigned char hv_get_nmi_reason(void)
 	return 0;
 }
 
+#ifdef CONFIG_X86_LOCAL_APIC
+/*
+ * Prior to WS2016 Debug-VM sends NMIs to all CPUs which makes
+ * it dificult to process CHANNELMSG_UNLOAD in case of crash. Handle
+ * unknown NMI on the first CPU which gets it.
+ */
+static int hv_nmi_unknown(unsigned int val, struct pt_regs *regs)
+{
+	static atomic_t nmi_cpu = ATOMIC_INIT(-1);
+
+	if (!unknown_nmi_panic)
+		return NMI_DONE;
+
+	if (atomic_cmpxchg(&nmi_cpu, -1, raw_smp_processor_id()) != -1)
+		return NMI_HANDLED;
+
+	return NMI_DONE;
+}
+#endif
+
 static void __init ms_hyperv_init_platform(void)
 {
 	/*
@@ -183,6 +204,9 @@ static void __init ms_hyperv_init_platform(void)
 		pr_info("HyperV: LAPIC Timer Frequency: %#x\n",
 			lapic_timer_frequency);
 	}
+
+	register_nmi_handler(NMI_UNKNOWN, hv_nmi_unknown, NMI_FLAG_FIRST,
+			     "hv_nmi_unknown");
 #endif
 
 	if (ms_hyperv.features & HV_X64_MSR_TIME_REF_COUNT_AVAILABLE)
-- 
2.9.3