x86/hyperv: Avoid reporting bogus NMI status for Gen2 instances

Generation2 instances don't support reporting the NMI status on port 0x61,
read from there returns 'ff' and we end up reporting nonsensical PCI
error (as there is no PCI bus in these instances) on all NMIs:

    NMI: PCI system error (SERR) for reason ff on CPU 0.
    Dazed and confused, but trying to continue

Fix the issue by overriding x86_platform.get_nmi_reason. Use 'booted on
EFI' flag to detect Gen2 instances.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Cathy Avery <cavery@redhat.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: devel@linuxdriverproject.org
Link: http://lkml.kernel.org/r/1460728232-31433-1-git-send-email-vkuznets@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/mshyperv.c

@@ -152,6 +152,11 @@ static struct clocksource hyperv_cs = {
 	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
 };
 
+static unsigned char hv_get_nmi_reason(void)
+{
+	return 0;
+}
+
 static void __init ms_hyperv_init_platform(void)
 {
 	/*
@@ -191,6 +196,13 @@ static void __init ms_hyperv_init_platform(void)
 	machine_ops.crash_shutdown = hv_machine_crash_shutdown;
 #endif
 	mark_tsc_unstable("running on Hyper-V");
+
+	/*
+	 * Generation 2 instances don't support reading the NMI status from
+	 * 0x61 port.
+	 */
+	if (efi_enabled(EFI_BOOT))
+		x86_platform.get_nmi_reason = hv_get_nmi_reason;
 }
 
 const __refconst struct hypervisor_x86 x86_hyper_ms_hyperv = {