[Xen-devel] [PATCH 6/6] x86: introduce dr_mask_idx() helper function...

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

...to avoid repeated open-coding.

Unfortunately the mapping from MSR index MSR_AMD64_DR<idx>_ADDRESS_MASK
to the 'idx' value is non-trivial since the MSR index corresponding to
idx value 0 is non-consecutive with the MSR indices corresponding to
idx values 1-3. This mapping is currently dealt with by near-identical
open coding in guest_rdmsr() and guest_wrmsr().

This patch adds a helper function, dr_mask_idx(), to handle the mapping
and then uses this in guest_rdmsr() and guest_wrmsr() instead, thus
making the code somewhat neater.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
---
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/msr.c        | 18 +++++-------------
 xen/include/asm-x86/msr.h | 21 +++++++++++++++++++++
 2 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 3aa79031cf..4e4c9c805a 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -21,7 +21,6 @@
 
 #include <xen/init.h>
 #include <xen/lib.h>
-#include <xen/nospec.h>
 #include <xen/sched.h>
 
 #include <asm/debugreg.h>
@@ -121,7 +120,7 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val)
     const struct domain *d = v->domain;
     const struct cpuid_policy *cp = d->arch.cpuid;
     const struct msr_policy *mp = d->arch.msr;
-    const struct vcpu_msrs *msrs = v->arch.msrs;
+    struct vcpu_msrs *msrs = v->arch.msrs;
     int ret = X86EMUL_OKAY;
 
     switch ( msr )
@@ -202,13 +201,10 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t 
*val)
          */
 #ifdef CONFIG_HVM
         if ( v == current && is_hvm_domain(d) && v->arch.hvm.flag_dr_dirty )
-            rdmsrl(msr, *val);
-        else
+            rdmsrl(msr, msrs->dr_mask[dr_mask_idx(msr)]);
 #endif
-            *val = msrs->dr_mask[
-                array_index_nospec((msr == MSR_AMD64_DR0_ADDRESS_MASK)
-                                   ? 0 : (msr - MSR_AMD64_DR1_ADDRESS_MASK + 
1),
-                                   ARRAY_SIZE(msrs->dr_mask))];
+
+        *val = msrs->dr_mask[dr_mask_idx(msr)];
         break;
 
     default:
@@ -377,11 +373,7 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
         if ( !cp->extd.dbext || val != (uint32_t)val )
             goto gp_fault;
 
-        msrs->dr_mask[
-            array_index_nospec((msr == MSR_AMD64_DR0_ADDRESS_MASK)
-                               ? 0 : (msr - MSR_AMD64_DR1_ADDRESS_MASK + 1),
-                               ARRAY_SIZE(msrs->dr_mask))] = val;
-
+        msrs->dr_mask[dr_mask_idx(msr)] = val;
         if ( v == curr && (curr->arch.dr7 & DR7_ACTIVE_MASK) )
             wrmsrl(msr, val);
         break;
diff --git a/xen/include/asm-x86/msr.h b/xen/include/asm-x86/msr.h
index e9c685613e..5eb5e6dace 100644
--- a/xen/include/asm-x86/msr.h
+++ b/xen/include/asm-x86/msr.h
@@ -5,6 +5,7 @@
 
 #ifndef __ASSEMBLY__
 
+#include <xen/nospec.h>
 #include <xen/types.h>
 #include <xen/percpu.h>
 #include <xen/errno.h>
@@ -317,6 +318,26 @@ struct vcpu_msrs
     } xss;
 };
 
+static inline unsigned int dr_mask_idx(uint32_t msr)
+{
+    switch (msr)
+    {
+    default:
+        ASSERT_UNREACHABLE();
+        /* Fallthrough */
+    case MSR_AMD64_DR0_ADDRESS_MASK:
+        return 0;
+
+    case MSR_AMD64_DR1_ADDRESS_MASK ... MSR_AMD64_DR3_ADDRESS_MASK:
+    {
+        struct vcpu_msrs msrs; /* only used for ARRAY_SIZE() */
+
+        return array_index_nospec(msr - MSR_AMD64_DR1_ADDRESS_MASK + 1,
+                                  ARRAY_SIZE(msrs.dr_mask));
+    }
+    }
+}
+
 void init_guest_msr_policy(void);
 int init_domain_msr_policy(struct domain *d);
 int init_vcpu_msr_policy(struct vcpu *v);
-- 
2.20.1


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