There is a feature in Hyper-V (Debug-VM --InjectNonMaskableInterrupt) which
injects NMI to the guest. Prior to WS2016 the NMI is injected to all CPUs
of the guest and WS2016 injects it to CPU0 only. When unknown_nmi_panic is
enabled and we'd like to do kdump we need to perform some minimal cleanup
so the kdump kernel will be able to initialize VMBus devices, this cleanup
includes sending CHANNELMSG_UNLOAD to the host waiting for
CHANNELMSG_UNLOAD_RESPONSE to arrive. WS2012R2 always sends the response
to the CPU which was used to send CHANNELMSG_REQUESTOFFERS on VMBus module
load and not to the CPU which is sending CHANNELMSG_UNLOAD. As we can't do
any cross-CPU work reliably on crash we have vmbus_wait_for_unload()
function which tries to read CHANNELMSG_UNLOAD_RESPONSE on all CPUs message
pages and this sometimes works. It was discovered that in case the host
wants to send more than one message to a secondary CPU (not the CPU running
vmbus_wait_for_unload()) we're unable to get it as after reading the first
message we're supposed to do EOMing by doing wrmsrl(HV_X64_MSR_EOM, 0) but
this is per-CPU. I have a feeling that this was working some time ago when
I implemented vmbus_wait_for_unload(), the host was re-trying to deliver a
message even without wrmsrl() but apparently this doesn't work any more.
Unfortunately there is not that much we can do when all CPUs get NMI as
all but the first one are getting blocked with interrupts disabled. What we
can do is limit processing unknown interrupts to the first CPU which gets
it in case we're about to crash.

Signed-off-by: Vitaly Kuznetsov <>
Acked-by: K. Y. Srinivasan <>
Changes since v1:
- Put everything under #ifdef CONFIG_X86_LOCAL_APIC as unknown_nmi_panic is
  otherwise undefined. [kbuild test robot]
- Add KY's Ack (hopefully stands with the above mentioned change)
 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..34b29f0 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;
@@ -158,6 +159,26 @@ static unsigned char hv_get_nmi_reason(void)
        return 0;
+ * 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;
 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",
+       register_nmi_handler(NMI_LOCAL, hv_nmi_unknown, NMI_FLAG_FIRST,
+                            "hv_nmi_unknown");
        if (ms_hyperv.features & HV_X64_MSR_TIME_REF_COUNT_AVAILABLE)

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