xen/pv: support selecting safe/unsafe msr accesses

Instead of always doing the safe variants for reading and writing MSRs
in Xen PV guests, make the behavior controllable via Kconfig option
and a boot parameter.

The default will be the current behavior, which is to always use the
safe variant.
Signed-off-by: Juergen Gross <jgross@suse.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -6836,6 +6836,12 @@
 			Crash from Xen panic notifier, without executing late
 			panic() code such as dumping handler.
 
+	xen_msr_safe=	[X86,XEN]
+			Format: <bool>
+			Select whether to always use non-faulting (safe) MSR
+			access functions when running as Xen PV guest. The
+			default value is controlled by CONFIG_XEN_PV_MSR_SAFE.
+
 	xen_nopvspin	[X86,XEN]
 			Disables the qspinlock slowpath using Xen PV optimizations.
 			This parameter is obsoleted by "nopvspin" parameter, which

arch/x86/xen/Kconfig

@@ -92,3 +92,12 @@ config XEN_DOM0
 	select X86_X2APIC if XEN_PVH && X86_64
 	help
 	  Support running as a Xen Dom0 guest.
+
+config XEN_PV_MSR_SAFE
+	bool "Always use safe MSR accesses in PV guests"
+	default y
+	depends on XEN_PV
+	help
+	  Use safe (not faulting) MSR access functions even if the MSR access
+	  should not fault anyway.
+	  The default can be changed by using the "xen_msr_safe" boot parameter.

arch/x86/xen/enlighten_pv.c

@@ -108,6 +108,16 @@ struct tls_descs {
  */
 static DEFINE_PER_CPU(struct tls_descs, shadow_tls_desc);
 
+static __read_mostly bool xen_msr_safe = IS_ENABLED(CONFIG_XEN_PV_MSR_SAFE);
+
+static int __init parse_xen_msr_safe(char *str)
+{
+	if (str)
+		return strtobool(str, &xen_msr_safe);
+	return -EINVAL;
+}
+early_param("xen_msr_safe", parse_xen_msr_safe);
+
 static void __init xen_pv_init_platform(void)
 {
 	/* PV guests can't operate virtio devices without grants. */
@@ -1010,22 +1020,16 @@ static int xen_write_msr_safe(unsigned int msr, unsigned int low,
 
 static u64 xen_read_msr(unsigned int msr)
 {
-	/*
-	 * This will silently swallow a #GP from RDMSR.  It may be worth
-	 * changing that.
-	 */
 	int err;
 
-	return xen_read_msr_safe(msr, &err);
+	return xen_do_read_msr(msr, xen_msr_safe ? &err : NULL);
 }
 
 static void xen_write_msr(unsigned int msr, unsigned low, unsigned high)
 {
-	/*
-	 * This will silently swallow a #GP from WRMSR.  It may be worth
-	 * changing that.
-	 */
-	xen_write_msr_safe(msr, low, high);
+	int err;
+
+	xen_do_write_msr(msr, low, high, xen_msr_safe ? &err : NULL);
 }
 
 /* This is called once we have the cpu_possible_mask */