x86/traps: Fix error handling of the pv %dr7 shadow state

c/s "x86/pv: Introduce and use x86emul_write_dr()" fixed a bug with IO shadow
handling, in that it remained stale and visible until %dr7.L/G got set again.

However, it neglected the -EPERM return inbetween these two hunks, introducing
a different bug in which a write to %dr7 which tries to set IO breakpoints
without %cr4.DE being set clobbers the IO state, rather than leaves it alone.

Instead, move the zeroing slightly later, which guarentees that the shadow
gets written exactly once, on a successful update to %dr7.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 8a99174b67964ed981fb3506393b4e71faf972b9..e79ca883d73704df80e8eb01db634d9ffef0b8be 100644 (file)
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2123,9 +2123,6 @@ long set_debugreg(struct vcpu *v, unsigned int reg, unsigned long value)
         if ( value & DR_GENERAL_DETECT )
             return -EPERM;
 
-        /* Zero the IO shadow before recalculating the real %dr7 */
-        v->arch.debugreg[5] = 0;
-
         /* DR7.{G,L}E = 0 => debugging disabled for this domain. */
         if ( value & DR7_ACTIVE_MASK )
         {
@@ -2154,6 +2151,10 @@ long set_debugreg(struct vcpu *v, unsigned int reg, unsigned long value)
                  !(v->arch.debugreg[7] & DR7_ACTIVE_MASK) )
                 activate_debugregs(v);
         }
+        else
+            /* Zero the emulated controls if %dr7 isn't active. */
+            v->arch.debugreg[5] = 0;
+
         if ( v == curr )
             write_debugreg(7, value);
         break;