diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 8d3d52034a6d..d05ee0da55c5 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1088,7 +1088,7 @@ int arch_set_info_guest(
     if ( is_pv_domain(d) )
     {
         for ( i = 0; i < ARRAY_SIZE(v->arch.dr); i++ )
-            if ( !access_ok(c(debugreg[i]), sizeof(long)) )
+            if ( !breakpoint_addr_ok(c(debugreg[i])) )
                 return -EINVAL;
         /*
          * Prior to Xen 4.11, dr5 was used to hold the emulated-only
diff --git a/xen/arch/x86/include/asm/debugreg.h b/xen/arch/x86/include/asm/debugreg.h
index 39ba312b84ee..b6454cc04e71 100644
--- a/xen/arch/x86/include/asm/debugreg.h
+++ b/xen/arch/x86/include/asm/debugreg.h
@@ -76,6 +76,25 @@
     __val;                                                  \
 })
 
+/*
+ * Architecturally, %dr{0..3} can have any arbitrary value.  However, Xen
+ * can't allow the guest to breakpoint the Xen address range, so we limit the
+ * guest to the lower canonical half, or above the Xen range in the higher
+ * canonical half.
+ *
+ * Breakpoint lengths are specified to mask the low order address bits,
+ * meaning all breakpoints are naturally aligned.  With %dr7, the widest
+ * breakpoint is 8 bytes.  With DBEXT, the widest breakpoint is 4G.  Both of
+ * the Xen boundaries have >4G alignment.
+ *
+ * In principle we should account for HYPERVISOR_COMPAT_VIRT_START(d), but
+ * 64bit Xen has never enforced this for compat guests, and there's no problem
+ * (to Xen) if the guest breakpoints it's alias of the M2P.  Skipping this
+ * aspect simplifies the logic, and causes us not to reject a migrating guest
+ * which operated fine on prior versions of Xen.
+ */
+#define breakpoint_addr_ok(a) __addr_ok(a)
+
 struct vcpu;
 long set_debugreg(struct vcpu *, unsigned int reg, unsigned long value);
 void activate_debugregs(const struct vcpu *);
diff --git a/xen/arch/x86/pv/misc-hypercalls.c b/xen/arch/x86/pv/misc-hypercalls.c
index 99f502812868..b529f00ea127 100644
--- a/xen/arch/x86/pv/misc-hypercalls.c
+++ b/xen/arch/x86/pv/misc-hypercalls.c
@@ -61,7 +61,7 @@ long set_debugreg(struct vcpu *v, unsigned int reg, unsigned long value)
     switch ( reg )
     {
     case 0 ... 3:
-        if ( !access_ok(value, sizeof(long)) )
+        if ( !breakpoint_addr_ok(value) )
             return -EPERM;
 
         v->arch.dr[reg] = value;