Drivers: HV: Send one page worth of kmsg dump over Hyper-V during panic

In the VM mode on Hyper-V, currently, when the kernel panics, an error
code and few register values are populated in an MSR and the Hypervisor
notified. This information is collected on the host. The amount of
information currently collected is found to be limited and not very
actionable. To gather more actionable data, such as stack trace, the
proposal is to write one page worth of kmsg data on an allocated page
and the Hypervisor notified of the page address through the MSR.

- Sysctl option to control the behavior, with ON by default.

Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Sunil Muthuswamy <sunilmut@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/sysctl/kernel.txt

@@ -39,6 +39,7 @@ show up in /proc/sys/kernel:
 - hung_task_check_count
 - hung_task_timeout_secs
 - hung_task_warnings
+- hyperv_record_panic_msg
 - kexec_load_disabled
 - kptr_restrict
 - l2cr                        [ PPC only ]
@@ -374,6 +375,16 @@ This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
 
 ==============================================================
 
+hyperv_record_panic_msg:
+
+Controls whether the panic kmsg data should be reported to Hyper-V.
+
+0: do not report panic kmsg data.
+
+1: report the panic kmsg data. This is the default behavior.
+
+==============================================================
+
 kexec_load_disabled:
 
 A toggle indicating if the kexec_load syscall has been disabled. This