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>

1 files changed, 110 insertions, 0 deletions

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index b10fe26c4891..05e37283d7c3 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -56,6 +56,8 @@ static struct completion probe_event;
 
 static int hyperv_cpuhp_online;
 
+static void *hv_panic_page;
+
 static int hyperv_panic_event(struct notifier_block *nb, unsigned long val,
 			      void *args)
 {
@@ -1018,6 +1020,75 @@ static void vmbus_isr(void)
 	add_interrupt_randomness(HYPERVISOR_CALLBACK_VECTOR, 0);
 }
 
+/*
+ * Boolean to control whether to report panic messages over Hyper-V.
+ *
+ * It can be set via /proc/sys/kernel/hyperv/record_panic_msg
+ */
+static int sysctl_record_panic_msg = 1;
+
+/*
+ * Callback from kmsg_dump. Grab as much as possible from the end of the kmsg
+ * buffer and call into Hyper-V to transfer the data.
+ */
+static void hv_kmsg_dump(struct kmsg_dumper *dumper,
+			 enum kmsg_dump_reason reason)
+{
+	size_t bytes_written;
+	phys_addr_t panic_pa;
+
+	/* We are only interested in panics. */
+	if ((reason != KMSG_DUMP_PANIC) || (!sysctl_record_panic_msg))
+		return;
+
+	panic_pa = virt_to_phys(hv_panic_page);
+
+	/*
+	 * Write dump contents to the page. No need to synchronize; panic should
+	 * be single-threaded.
+	 */
+	if (!kmsg_dump_get_buffer(dumper, true, hv_panic_page,
+				  PAGE_SIZE, &bytes_written)) {
+		pr_err("Hyper-V: Unable to get kmsg data for panic\n");
+		return;
+	}
+
+	hyperv_report_panic_msg(panic_pa, bytes_written);
+}
+
+static struct kmsg_dumper hv_kmsg_dumper = {
+	.dump = hv_kmsg_dump,
+};
+
+static struct ctl_table_header *hv_ctl_table_hdr;
+static int zero;
+static int one = 1;
+
+/*
+ * sysctl option to allow the user to control whether kmsg data should be
+ * reported to Hyper-V on panic.
+ */
+static struct ctl_table hv_ctl_table[] = {
+	{
+		.procname       = "hyperv_record_panic_msg",
+		.data           = &sysctl_record_panic_msg,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one
+	},
+	{}
+};
+
+static struct ctl_table hv_root_table[] = {
+	{
+		.procname	= "kernel",
+		.mode		= 0555,
+		.child		= hv_ctl_table
+	},
+	{}
+};
 
 /*
  * vmbus_bus_init -Main vmbus driver initialization routine.
@@ -1065,6 +1136,32 @@ static int vmbus_bus_init(void)
 	 * Only register if the crash MSRs are available
 	 */
 	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
+		u64 hyperv_crash_ctl;
+		/*
+		 * Sysctl registration is not fatal, since by default
+		 * reporting is enabled.
+		 */
+		hv_ctl_table_hdr = register_sysctl_table(hv_root_table);
+		if (!hv_ctl_table_hdr)
+			pr_err("Hyper-V: sysctl table register error");
+
+		/*
+		 * Register for panic kmsg callback only if the right
+		 * capability is supported by the hypervisor.
+		 */
+		rdmsrl(HV_X64_MSR_CRASH_CTL, hyperv_crash_ctl);
+		if (hyperv_crash_ctl & HV_CRASH_CTL_CRASH_NOTIFY_MSG) {
+			hv_panic_page = (void *)get_zeroed_page(GFP_KERNEL);
+			if (hv_panic_page) {
+				ret = kmsg_dump_register(&hv_kmsg_dumper);
+				if (ret)
+					pr_err("Hyper-V: kmsg dump register "
+						"error 0x%x\n", ret);
+			} else
+				pr_err("Hyper-V: panic message page memory "
+					"allocation failed");
+		}
+
 		register_die_notifier(&hyperv_die_block);
 		atomic_notifier_chain_register(&panic_notifier_list,
 					       &hyperv_panic_block);
@@ -1081,6 +1178,11 @@ err_alloc:
 	hv_remove_vmbus_irq();
 
 	bus_unregister(&hv_bus);
+	free_page((unsigned long)hv_panic_page);
+	if (!hv_ctl_table_hdr) {
+		unregister_sysctl_table(hv_ctl_table_hdr);
+		hv_ctl_table_hdr = NULL;
+	}
 
 	return ret;
 }
@@ -1785,10 +1887,18 @@ static void __exit vmbus_exit(void)
 	vmbus_free_channels();
 
 	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
+		kmsg_dump_unregister(&hv_kmsg_dumper);
 		unregister_die_notifier(&hyperv_die_block);
 		atomic_notifier_chain_unregister(&panic_notifier_list,
 						 &hyperv_panic_block);
 	}
+
+	free_page((unsigned long)hv_panic_page);
+	if (!hv_ctl_table_hdr) {
+		unregister_sysctl_table(hv_ctl_table_hdr);
+		hv_ctl_table_hdr = NULL;
+	}
+
 	bus_unregister(&hv_bus);
 
 	cpuhp_remove_state(hyperv_cpuhp_online);