Re: [PATCH V4 01/20] rv: Add Runtime Verification (RV) interface

From: Tao Zhou <tao.zhou@linux.dev>
To: Matthew Wilcox <willy@infradead.org>
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, Tao Zhou <tao.zhou@linux.dev>
Subject: Re: [PATCH V4 01/20] rv: Add Runtime Verification (RV) interface
Date: Fri, 8 Jul 2022 23:36:06 +0800	[thread overview]
Message-ID: <YshO5tSeNyFpNFc4@geo.homenetwork> (raw)
In-Reply-To: <YsXMEeqGr2vdJCa7@casper.infradead.org>

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.