Re: [Qemu-devel] [PATCH 0/8] target-i386: Implement debug extensions

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/09/2015 20:45, Richard Henderson wrote:
> Best guess, since I can't find any code that actually uses them.
> Linux actively turns them off at boot...

I've sent a kvm-unit-tests patch to test debug extensions.  It shows
that debug extensions work, but the following needs to be squashed in
patch 4:

diff --git a/target-i386/bpt_helper.c b/target-i386/bpt_helper.c
index c258598..b24e446 100644
--- a/target-i386/bpt_helper.c
+++ b/target-i386/bpt_helper.c
@@ -134,14 +134,14 @@ void cpu_x86_update_dr7(CPUX86State *env, uint32_t new_dr7)
         int mod = ((old_dr7 | old_dr7 * 2) ^ (new_dr7 | new_dr7 * 2)) & 0xff;
 
         for (i = 0; i < DR7_MAX_BP; i++) {
-            if (mod & (2 << i * 2)) {
-                /* We know that register i has changed enable state;
-                   recheck what that state should be and apply.  */
-                if (hw_breakpoint_enabled(new_dr7, i)) {
-                    iobpt |= hw_breakpoint_insert(env, i);
-                } else {
-                    hw_breakpoint_remove(env, i);
-                }
+            if ((mod & (2 << i * 2)) && !hw_breakpoint_enabled(new_dr7, i)) {
+                hw_breakpoint_remove(env, i);
+            }
+        }
+        env->dr[7] = new_dr7 | DR7_FIXED_1;
+        for (i = 0; i < DR7_MAX_BP; i++) {
+            if (mod & (2 << i * 2) && hw_breakpoint_enabled(new_dr7, i)) {
+                iobpt |= hw_breakpoint_insert(env, i);
             } else if (hw_breakpoint_type(new_dr7, i) == DR7_TYPE_IO_RW
                        && hw_breakpoint_enabled(new_dr7, i)) {
                 iobpt |= HF_IOBPT_MASK;

Otherwise, hw_breakpoint_insert doesn't work because it expects to
see an updated env->dr[7].

There are a couple other issues that the tests expose, but they are
not regressions so I will send patches later.

Paolo