[PATCH v4 2/5] x86/pv: allow reading FEATURE_CONTROL MSR

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Linux PV guests will attempt to read the FEATURE_CONTROL MSR, so move
the handling done in VMX code into guest_rdmsr as it can be shared
between PV and HVM guests that way.

Note that there's a slight behavior change and attempting to read the
MSR when no features are available will result in a fault.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v3:
 - Only allow reading the MSR when there are bits available (different
   than bit 0).

Changes from v1:
 - Move the VMX implementation into guest_rdmsr.
---
 xen/arch/x86/hvm/vmx/vmx.c |  8 +-------
 xen/arch/x86/msr.c         | 12 ++++++++++++
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index c4b40bf3cb..709ea149d1 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2980,13 +2980,7 @@ static int vmx_msr_read_intercept(unsigned int msr, 
uint64_t *msr_content)
     case MSR_IA32_DEBUGCTLMSR:
         __vmread(GUEST_IA32_DEBUGCTL, msr_content);
         break;
-    case MSR_IA32_FEATURE_CONTROL:
-        *msr_content = IA32_FEATURE_CONTROL_LOCK;
-        if ( vmce_has_lmce(curr) )
-            *msr_content |= IA32_FEATURE_CONTROL_LMCE_ON;
-        if ( nestedhvm_enabled(curr->domain) )
-            *msr_content |= IA32_FEATURE_CONTROL_ENABLE_VMXON_OUTSIDE_SMX;
-        break;
+
     case MSR_IA32_VMX_BASIC...MSR_IA32_VMX_VMFUNC:
         if ( !nvmx_msr_read_intercept(msr, msr_content) )
             goto gp_fault;
diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 74bf7d9589..79fbb9e940 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -25,6 +25,7 @@
 #include <xen/sched.h>
 
 #include <asm/debugreg.h>
+#include <asm/hvm/nestedhvm.h>
 #include <asm/hvm/viridian.h>
 #include <asm/msr.h>
 #include <asm/setup.h>
@@ -197,6 +198,17 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t 
*val)
         /* Not offered to guests. */
         goto gp_fault;
 
+    case MSR_IA32_FEATURE_CONTROL:
+        if ( !cp->basic.vmx && !vmce_has_lmce(v) )
+            goto gp_fault;
+
+        *val = IA32_FEATURE_CONTROL_LOCK;
+        if ( vmce_has_lmce(v) )
+            *val |= IA32_FEATURE_CONTROL_LMCE_ON;
+        if ( cp->basic.vmx )
+            *val |= IA32_FEATURE_CONTROL_ENABLE_VMXON_OUTSIDE_SMX;
+        break;
+
     case MSR_IA32_PLATFORM_ID:
         if ( !(cp->x86_vendor & X86_VENDOR_INTEL) ||
              !(boot_cpu_data.x86_vendor & X86_VENDOR_INTEL) )
-- 
2.28.0