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[Xen-devel] [PATCH] x86/hvm: Conditionally leave CPUID Faulting active in HVM context



If the hardware supports faulting, and the guest has chosen to use it, leave
faulting active in HVM context.

It is more efficient to have hardware convert CPUID to a #GP fault (which we
don't intercept), than to take a VMExit and have Xen re-inject a #GP fault.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>
---
 xen/arch/x86/cpu/intel.c   |  5 +++--
 xen/arch/x86/hvm/vmx/vmx.c | 12 ++++++++++--
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/cpu/intel.c b/xen/arch/x86/cpu/intel.c
index 2e11662..d0e380c 100644
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -175,8 +175,9 @@ static void intel_ctxt_switch_levelling(const struct vcpu 
*next)
                 * generating the maximum full cpuid policy into Xen, at which
                 * this problem will disappear.
                 */
-               set_cpuid_faulting(nextd && is_pv_domain(nextd) &&
-                                  !is_control_domain(nextd));
+               set_cpuid_faulting(nextd && !is_control_domain(nextd) &&
+                                  (is_pv_domain(nextd) ||
+                                   next->arch.cpuid_faulting));
                return;
        }
 
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 61925cf..19294cb 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2866,11 +2866,19 @@ static int vmx_msr_write_intercept(unsigned int msr, 
uint64_t msr_content)
         break;
 
     case MSR_INTEL_MISC_FEATURES_ENABLES:
+    {
+        bool old_cpuid_faulting = v->arch.cpuid_faulting;
+
         if ( msr_content & ~MSR_MISC_FEATURES_CPUID_FAULTING )
             goto gp_fault;
-        v->arch.cpuid_faulting =
-            !!(msr_content & MSR_MISC_FEATURES_CPUID_FAULTING);
+
+        v->arch.cpuid_faulting = msr_content & 
MSR_MISC_FEATURES_CPUID_FAULTING;
+
+        if ( cpu_has_cpuid_faulting &&
+             (old_cpuid_faulting ^ v->arch.cpuid_faulting) )
+            ctxt_switch_levelling(v);
         break;
+    }
 
     default:
         if ( passive_domain_do_wrmsr(msr, msr_content) )
-- 
2.1.4


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