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

From: Corneliu ZUZU <czuzu@bitdefender.com>
To: Tamas K Lengyel <tamas@tklengyel.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Razvan Cojocaru <rcojocaru@bitdefender.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Xen-devel <xen-devel@lists.xen.org>,
	Julien Grall <julien.grall@arm.com>,
	Paul Durrant <paul.durrant@citrix.com>,
	Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH 11/16] x86/monitor: fix: treat -monitor- properly, as a subsys of the vm-event subsys
Date: Mon, 11 Jul 2016 23:20:44 +0300	[thread overview]
Message-ID: <2c2b48d6-c0d8-71c8-8bb6-47690ce5e959@bitdefender.com> (raw)
In-Reply-To: <CABfawh=pONBB3vBtD2xfyNC5UK20_DxE1hWcURrDRoJ9vC6Wzw@mail.gmail.com>

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.

Corneliu.

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