KVM: x86: Fix DR7 mask on task-switch while debugging

If the host sets hardware breakpoints to debug the guest, and a task-switch
occurs in the guest, the architectural DR7 will not be updated. The effective
DR7 would be updated instead.

This fix puts the DR7 update during task-switch emulation, so it now uses the
standard DR setting mechanism instead of the one that was previously used. As a
bonus, the update of DR7 will now be effective for AMD as well.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -25,6 +25,7 @@
 #include <linux/module.h>
 #include <asm/kvm_emulate.h>
 #include <linux/stringify.h>
+#include <asm/debugreg.h>
 
 #include "x86.h"
 #include "tss.h"
@@ -2849,7 +2850,7 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
 	ulong old_tss_base =
 		ops->get_cached_segment_base(ctxt, VCPU_SREG_TR);
 	u32 desc_limit;
-	ulong desc_addr;
+	ulong desc_addr, dr7;
 
 	/* FIXME: old_tss_base == ~0 ? */
 
@@ -2934,6 +2935,9 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
 		ret = em_push(ctxt);
 	}
 
+	ops->get_dr(ctxt, 7, &dr7);
+	ops->set_dr(ctxt, 7, dr7 & ~(DR_LOCAL_ENABLE_MASK | DR_LOCAL_SLOWDOWN));
+
 	return ret;
 }
 

arch/x86/kvm/vmx.c

@@ -5708,9 +5708,6 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
 		return 0;
 	}
 
-	/* clear all local breakpoint enable flags */
-	vmcs_writel(GUEST_DR7, vmcs_readl(GUEST_DR7) & ~0x155);
-
 	/*
 	 * TODO: What about debug traps on tss switch?
 	 *       Are we supposed to inject them and update dr6?