Re: [Xen-devel] [PATCH 11/16] x86/monitor: fix: treat -monitor- properly, as a subsys of the vm-event subsys

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

On 7/12/2016 12:27 AM, Tamas K Lengyel wrote:

On Mon, Jul 11, 2016 at 2:20 PM, Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx> wrote:

On 7/11/2016 7:38 PM, Tamas K Lengyel wrote:

diff --git a/xen/include/asm-arm/monitor.h
b/xen/include/asm-arm/monitor.h
index 9a9734a..7ef30f1 100644

[snip]

I keep seeing '[snip]' lately but I don't know what it means.

Placeholder for "I've cut some of your patch content here to shorten
the message I'm sending".

diff --git a/xen/include/xen/monitor.h b/xen/include/xen/monitor.h
index 2171d04..605caf0 100644
--- a/xen/include/xen/monitor.h
+++ b/xen/include/xen/monitor.h
@@ -22,12 +22,15 @@
   #ifndef __XEN_MONITOR_H__
   #define __XEN_MONITOR_H__

-#include <public/vm_event.h>
-
-struct domain;
-struct xen_domctl_monitor_op;
+#include <xen/sched.h>

   int monitor_domctl(struct domain *d, struct xen_domctl_monitor_op
*op);
+
+static inline bool_t monitor_domain_initialised(const struct domain
*d)
+{
+    return d->monitor.initialised;

This should be !!d->monitor.initialised.

It's the value of a bit, thus should be 0 or 1. Am I missing something?

Using a bitfield is effectively doing a bitmask for the bit(s).
However, returning the value after applying a bitmask is not
necessarily 0/1 (ie. bool_t). For example I ran into problems with
this in the past (see
https://lists.xen.org/archives/html/xen-devel/2015-08/msg01948.html).

Tamas

The example you provided actually returns a value &-ed with a flag
(bitmask), of course it returns either 0 or the flag itself :-). AFAIK a
single-bit item in a bitfield (note a -bitfield-, not e.g. an unsigned int
&-ed with (1<<x)) will always be either 0 or 1.

I would assume as well but I'm not actually sure if that's guaranteed
or if it's only compiler defined behavior.

Tamas

As per http://en.cppreference.com/w/c/language/bit_field .

"The number of bits in a bit field (width) sets the limit to the range of values it can hold"

So if it's width is 1, it can either be 0 or 1.

Corneliu.

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