[Xen-devel] [PATCH] vmx: fix debugctl handling

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

I recently realized that the original way of dealing with the DebugCtl
MSR on VMX failed to make use of the dedicated guest VMCS field. This
is being fixed with this patch.

What is puzzling me to a certain degree is that while there is a guest
VMCS field for this MSR, there's no equivalent host load field, but
there's also no indication that the MSR would be cleared during a
vmexit. Can someone at Intel perhaps give a statement on this?

I would really be possible to avoid the intercept on DebugCtl reads
altogether, but that would require new support functions to
individually disable read and write intercepts (which currently are
always handled as a pair). I'm not sure such a change is warranted
for a debugging only feature.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

Index: 2008-03-05/xen/arch/x86/hvm/vmx/vmcs.c
===================================================================
--- 2008-03-05.orig/xen/arch/x86/hvm/vmx/vmcs.c 2008-02-18 10:07:45.000000000 
+0100
+++ 2008-03-05/xen/arch/x86/hvm/vmx/vmcs.c      2008-03-14 14:08:11.000000000 
+0100
@@ -870,7 +870,7 @@ void vmcs_dump_vcpu(struct vcpu *v)
     x  = (unsigned long long)vmr(TSC_OFFSET_HIGH) << 32;
     x |= (uint32_t)vmr(TSC_OFFSET);
     printk("TSC Offset = %016llx\n", x);
-    x  = (unsigned long long)vmr(GUEST_IA32_DEBUGCTL) << 32;
+    x  = (unsigned long long)vmr(GUEST_IA32_DEBUGCTL_HIGH) << 32;
     x |= (uint32_t)vmr(GUEST_IA32_DEBUGCTL);
     printk("DebugCtl=%016llx DebugExceptions=%016llx\n", x,
            (unsigned long long)vmr(GUEST_PENDING_DBG_EXCEPTIONS));
Index: 2008-03-05/xen/arch/x86/hvm/vmx/vmx.c
===================================================================
--- 2008-03-05.orig/xen/arch/x86/hvm/vmx/vmx.c  2008-02-26 10:43:52.000000000 
+0100
+++ 2008-03-05/xen/arch/x86/hvm/vmx/vmx.c       2008-03-14 14:14:40.000000000 
+0100
@@ -1512,8 +1512,10 @@ static int vmx_msr_read_intercept(struct
         msr_content = var_range_base[index];
         break;
     case MSR_IA32_DEBUGCTLMSR:
-        if ( vmx_read_guest_msr(v, ecx, &msr_content) != 0 )
-            msr_content = 0;
+        msr_content = __vmread(GUEST_IA32_DEBUGCTL);
+#ifdef __i386__
+        msr_content |= (u64)__vmread(GUEST_IA32_DEBUGCTL_HIGH) << 32;
+#endif
         break;
     case MSR_IA32_VMX_BASIC...MSR_IA32_VMX_PROCBASED_CTLS2:
         goto gp_fault;
@@ -1732,11 +1734,15 @@ static int vmx_msr_write_intercept(struc
         }
 
         if ( (rc < 0) ||
-             (vmx_add_guest_msr(v, ecx) < 0) ||
              (vmx_add_host_load_msr(v, ecx) < 0) )
             vmx_inject_hw_exception(v, TRAP_machine_check, 0);
         else
-            vmx_write_guest_msr(v, ecx, msr_content);
+        {
+            __vmwrite(GUEST_IA32_DEBUGCTL, msr_content);
+#ifdef __i386__
+            __vmwrite(GUEST_IA32_DEBUGCTL_HIGH, msr_content >> 32);
+#endif
+        }
 
         break;
     }




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