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Date:   Fri, 8 Jul 2022 16:55:44 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Tao Zhou <tao.zhou@linux.dev>
Cc:     Daniel Bristot de Oliveira <bristot@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Wim Van Sebroeck <wim@linux-watchdog.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Gabriele Paoloni <gpaoloni@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Clark Williams <williams@redhat.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-trace-devel@vger.kernel.org
Subject: Re: [PATCH V4 01/20] rv: Add Runtime Verification (RV) interface
Message-ID: <YshTgMJcVBPuiEUN@casper.infradead.org>
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On Fri, Jul 08, 2022 at 11:36:06PM +0800, Tao Zhou wrote:
> On Wed, Jul 06, 2022 at 06:53:21PM +0100, Matthew Wilcox wrote:
> 
> > On Thu, Jul 07, 2022 at 01:49:02AM +0800, Tao Zhou wrote:
> > > > +struct rv_monitor {
> > > > +	const char		*name;
> > > > +	const char		*description;
> > > > +	bool			enabled;
> > > 
> > > Can the 'bool enabled;' be put at the end like the definition of
> > > structure rv_monitor_def. If '8+8+sizeof(bool)+8+8+8' not the same
> > > as '8+8+8+8+8+sizeof(bool)', I mean is it possible that after the
> > > end of stucture there is a int or char not require to align to 8 as
> > > an example from my nonsense.
> > 
> > That will make no difference at all.  C doesn't allow other variables
> > to "fill in the hole" at the end of the structure like that.  For
> > example, one could legitimately do 'memset(&rvm, sizeof(rvm))',
> > and that would wipe out those other variables as well.
> 
> I mean if it is possible that if @enabled placed at the end of the
> structure rv_monitor will save some bytes.
> If @enabled place in between, the next function pointer which is 8 bytes
> will align to be in x8 address and the size of structure rv_monitor is
> larger than been placed at the end of the structure.
> Or the compiler can do magic that I can not guess.
> 
> Sorry for my late reply. I am not sure about this. But your reply is not
> about what I mean. You say that the size of structure is the same(I doute about this).
> But what my concert is that the other data next to the structure rv_monitor 
> how to align if placing the @enabled at the end.
> 
> Place in between, bytes:
> 8+8+8(padd)+8+8+8=48
> 
> Place at the end, bytes:
> 8+8+8+8+8+sizeof(bool)=?
> 
> ? is small than 48 and the data next to the data structure rv_monitor can use
> the saved byte which is the result of placing @enabled at the end of structure
> to place the data.

You don't need to take my word for it.  You can try it yourself.