Hi, Will,

Are you happy with this?:

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c

+bool hw_breakpoint_enabled(void)
+{
+    struct perf_event **slots;
+    int i;
+
+    slots = this_cpu_ptr(bp_on_reg);
+    for (i = 0; i < core_num_brps; i++) {
+        if (slots[i])
+            return true;
+    }
+
+    slots = this_cpu_ptr(wp_on_reg);
+    for (i = 0; i < core_num_wrps; i++) {
+        if (slots[i])
+            return true;
+    }
+
+    return false;
+}

It doesn't change any existing functions, and even doesn't add a new
variables, it just provide an indication for KVM, and it's low-overhead.

We will only call it upon guest entry, so there is also no race for it.


On July 7, 2015 6:24:06 PM GMT+08:00, Will Deacon [off-list ref] wrote:

On Tue, Jul 07, 2015 at 11:06:57AM +0100, Zhichao Huang wrote:

quoted

Chazy and me are talking about how to reduce the saving/restoring
overhead for debug registers.
We want to add a state in hw_breakpoint.c to indicate whether the

host

quoted

enable any hwbrpts or not (might export a fuction that kvm can call),
then we can read this state from memory instead of reading from real
hardware registers, and to decide whether we need a world switch or
not.
Does it acceptable?

Maybe, hard to tell without the code. There are obvious races to deal
with
if you use variables to indicate whether resources are in use -- why
not
just trap debug access from the host as well? Then you could keep track
of
the "owner" in kvm and trap accesses from everybody else.

Will

-- 
Zhichao Huang