x86/svm: Don't clear clean bits on every context switch

This is overly agressive.  The APM says:

  The hypervisor must clear the entire VMCB field to 0 for a guest, under the
  following circumstances:

    * The hypervisor executes the guest on a different CPU core than one used
      the last time that guest was executed.

Clear the cleanbits only when the VMCB is used on a different pCPU rather than
on every context switch to the vCPU.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/hvm/svm/svm.c

@@ -890,7 +890,6 @@ static void cf_check svm_ctxt_switch_from(struct vcpu *v)
 
 static void cf_check svm_ctxt_switch_to(struct vcpu *v)
 {
-    struct vmcb_struct *vmcb = v->arch.hvm.svm.vmcb;
     int cpu = smp_processor_id();
 
     /*
@@ -908,7 +907,6 @@ static void cf_check svm_ctxt_switch_to(struct vcpu *v)
 
     svm_restore_dr(v);
 
-    vmcb->cleanbits.raw = 0;
     svm_tsc_ratio_load(v);
 
     if ( cpu_has_msr_tsc_aux )
@@ -953,6 +951,7 @@ static void noreturn cf_check svm_do_resume(void)
         hvm_migrate_pirqs(v);
         /* Migrating to another ASID domain.  Request a new ASID. */
         hvm_asid_flush_vcpu(v);
+        vmcb->cleanbits.raw = 0;
     }
 
     if ( !vcpu_guestmode && !vlapic_hw_disabled(vlapic) )