hv: export current Hyper-V clocksource

As a preparation to implementing Hyper-V PTP device supporting
.getcrosststamp we need to export a reference to the current Hyper-V
clocksource in use (MSR or TSC page).
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NK. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/hyperv/hv_init.c

@@ -99,6 +99,9 @@ static struct clocksource hyperv_cs_msr = {
 };
 
 static void *hypercall_pg;
+struct clocksource *hyperv_cs;
+EXPORT_SYMBOL_GPL(hyperv_cs);
+
 /*
  * This function is to be invoked early in the boot sequence after the
  * hypervisor has been detected.
@@ -141,10 +144,10 @@ void hyperv_init(void)
 		union hv_x64_msr_hypercall_contents tsc_msr;
 
 		tsc_pg = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL);
-		if (!tsc_pg) {
-			clocksource_register_hz(&hyperv_cs_msr, NSEC_PER_SEC/100);
-			return;
-		}
+		if (!tsc_pg)
+			goto register_msr_cs;
+
+		hyperv_cs = &hyperv_cs_tsc;
 
 		rdmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
 
@@ -161,6 +164,8 @@ void hyperv_init(void)
 	 * the partition counter.
 	 */
 
+register_msr_cs:
+	hyperv_cs = &hyperv_cs_msr;
 	if (ms_hyperv.features & HV_X64_MSR_TIME_REF_COUNT_AVAILABLE)
 		clocksource_register_hz(&hyperv_cs_msr, NSEC_PER_SEC/100);
 }

arch/x86/include/asm/mshyperv.h

@@ -3,6 +3,7 @@
 
 #include <linux/types.h>
 #include <linux/interrupt.h>
+#include <linux/clocksource.h>
 #include <asm/hyperv.h>
 
 /*
@@ -168,6 +169,8 @@ void hv_setup_crash_handler(void (*handler)(struct pt_regs *regs));
 void hv_remove_crash_handler(void);
 
 #if IS_ENABLED(CONFIG_HYPERV)
+extern struct clocksource *hyperv_cs;
+
 void hyperv_init(void);
 void hyperv_report_panic(struct pt_regs *regs);
 bool hv_is_hypercall_page_setup(void);