[PATCH v3 3/4] xen/pv: refactor msr access functions to support safe and unsafe accesses

[Juergen Gross <jgross@xxxxxxxx>]

Refactor and rename xen_read_msr_safe() and xen_write_msr_safe() to
support both cases of MSR accesses, safe ones and potentially GP-fault
generating ones.

This will prepare to no longer swallow GPs silently in xen_read_msr()
and xen_write_msr().

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
V2:
- init val in xen_do_read_msr() to 0 (Jan Beulich)
---
 arch/x86/xen/enlighten_pv.c | 75 +++++++++++++++++++++++++++----------
 1 file changed, 56 insertions(+), 19 deletions(-)

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 9b1a58dda935..d5b0844a1b7c 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -917,14 +917,18 @@ static void xen_write_cr4(unsigned long cr4)
        native_write_cr4(cr4);
 }
 
-static u64 xen_read_msr_safe(unsigned int msr, int *err)
+static u64 xen_do_read_msr(unsigned int msr, int *err)
 {
-       u64 val;
+       u64 val = 0;    /* Avoid uninitialized value for safe variant. */
 
        if (pmu_msr_read(msr, &val, err))
                return val;
 
-       val = native_read_msr_safe(msr, err);
+       if (err)
+               val = native_read_msr_safe(msr, err);
+       else
+               val = native_read_msr(msr);
+
        switch (msr) {
        case MSR_IA32_APICBASE:
                val &= ~X2APIC_ENABLE;
@@ -933,23 +937,39 @@ static u64 xen_read_msr_safe(unsigned int msr, int *err)
        return val;
 }
 
-static int xen_write_msr_safe(unsigned int msr, unsigned low, unsigned high)
+static void set_seg(unsigned int which, unsigned int low, unsigned int high,
+                   int *err)
 {
-       int ret;
-       unsigned int which;
-       u64 base;
+       u64 base = ((u64)high << 32) | low;
+
+       if (HYPERVISOR_set_segment_base(which, base) == 0)
+               return;
 
-       ret = 0;
+       if (err)
+               *err = -EIO;
+       else
+               WARN(1, "Xen set_segment_base(%u, %llx) failed\n", which, base);
+}
 
+/*
+ * Support write_msr_safe() and write_msr() semantics.
+ * With err == NULL write_msr() semantics are selected.
+ * Supplying an err pointer requires err to be pre-initialized with 0.
+ */
+static void xen_do_write_msr(unsigned int msr, unsigned int low,
+                            unsigned int high, int *err)
+{
        switch (msr) {
-       case MSR_FS_BASE:               which = SEGBASE_FS; goto set;
-       case MSR_KERNEL_GS_BASE:        which = SEGBASE_GS_USER; goto set;
-       case MSR_GS_BASE:               which = SEGBASE_GS_KERNEL; goto set;
-
-       set:
-               base = ((u64)high << 32) | low;
-               if (HYPERVISOR_set_segment_base(which, base) != 0)
-                       ret = -EIO;
+       case MSR_FS_BASE:
+               set_seg(SEGBASE_FS, low, high, err);
+               break;
+
+       case MSR_KERNEL_GS_BASE:
+               set_seg(SEGBASE_GS_USER, low, high, err);
+               break;
+
+       case MSR_GS_BASE:
+               set_seg(SEGBASE_GS_KERNEL, low, high, err);
                break;
 
        case MSR_STAR:
@@ -965,11 +985,28 @@ static int xen_write_msr_safe(unsigned int msr, unsigned 
low, unsigned high)
                break;
 
        default:
-               if (!pmu_msr_write(msr, low, high, &ret))
-                       ret = native_write_msr_safe(msr, low, high);
+               if (!pmu_msr_write(msr, low, high, err)) {
+                       if (err)
+                               *err = native_write_msr_safe(msr, low, high);
+                       else
+                               native_write_msr(msr, low, high);
+               }
        }
+}
+
+static u64 xen_read_msr_safe(unsigned int msr, int *err)
+{
+       return xen_do_read_msr(msr, err);
+}
+
+static int xen_write_msr_safe(unsigned int msr, unsigned int low,
+                             unsigned int high)
+{
+       int err = 0;
+
+       xen_do_write_msr(msr, low, high, &err);
 
-       return ret;
+       return err;
 }
 
 static u64 xen_read_msr(unsigned int msr)
-- 
2.35.3