x86/vmx: correctly gather gs_shadow value for current vCPU

Currently the gs_shadow value is only cached when the vCPU is being scheduled
out by Xen. Reporting this (usually) stale value through vm_event is incorrect,
since it doesn't represent the actual state of the vCPU at the time the event
was recorded. This prevents vm_event subscribers from correctly finding kernel
structures in the guest when it is trapped while in ring3.

Refresh shadow_gs value when the context being saved is for the current vCPU.

Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>

xen/arch/x86/hvm/vmx/vmx.c

@@ -779,12 +779,18 @@ static void vmx_load_cpu_state(struct vcpu *v, struct hvm_hw_cpu *data)
 
 static void vmx_save_vmcs_ctxt(struct vcpu *v, struct hvm_hw_cpu *ctxt)
 {
+    if ( v == current )
+        vmx_save_guest_msrs(v);
+
     vmx_save_cpu_state(v, ctxt);
     vmx_vmcs_save(v, ctxt);
 }
 
 static int vmx_load_vmcs_ctxt(struct vcpu *v, struct hvm_hw_cpu *ctxt)
 {
+    /* Not currently safe to use in current context. */
+    ASSERT(v != current);
+
     vmx_load_cpu_state(v, ctxt);
 
     if ( vmx_vmcs_restore(v, ctxt) )