[PATCH v2 3/3] xen/pv: support selecting safe/unsafe msr accesses

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  6 +++++
 arch/x86/xen/Kconfig                          |  9 +++++++
 arch/x86/xen/enlighten_pv.c                   | 24 +++++++++++--------
 3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 426fa892d311..1bda9cf18fae 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/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
diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
index 85246dd9faa1..9b1ec5d8c99c 100644
--- a/arch/x86/xen/Kconfig
+++ b/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.
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index d5b0844a1b7c..daae454191f2 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/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. */
@@ -1011,22 +1021,16 @@ 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 */
-- 
2.35.3