x86/fsgsbase: Fix Xen PV support

On Xen PV, SWAPGS doesn't work.  Teach __rdfsbase_inactive() and
__wrgsbase_inactive() to use rdmsrl()/wrmsrl() on Xen PV.  The Xen
pvop code will understand this and issue the correct hypercalls.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/f07c08f178fe9711915862b656722a207cd52c28.1593192140.git.luto@kernel.org

arch/x86/kernel/process_64.c

@@ -163,9 +163,15 @@ static noinstr unsigned long __rdgsbase_inactive(void)
 
 	lockdep_assert_irqs_disabled();
 
+	if (!static_cpu_has(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		gsbase = rdgsbase();
 		native_swapgs();
+	} else {
+		instrumentation_begin();
+		rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
+		instrumentation_end();
+	}
 
 	return gsbase;
 }
@@ -182,9 +188,15 @@ static noinstr void __wrgsbase_inactive(unsigned long gsbase)
 {
 	lockdep_assert_irqs_disabled();
 
+	if (!static_cpu_has(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		wrgsbase(gsbase);
 		native_swapgs();
+	} else {
+		instrumentation_begin();
+		wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
+		instrumentation_end();
+	}
 }
 
 /*