From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Mon, 2 Mar 2015 18:41:04 +0100
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Message-ID: <20150302174104.GA1574@hermes.click-hack.org>
References: <54F0CF9E.1040707@siemens.com>
 <20150227202724.GL434@hermes.click-hack.org>
 <54F0D599.8090009@siemens.com>
 <20150227212130.GP434@hermes.click-hack.org>
 <20150227212439.GQ434@hermes.click-hack.org>
 <54F4A04A.9050600@siemens.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <54F4A04A.9050600@siemens.com>
Subject: Re: [Xenomai] [RFC] Consolidated exception prologue/epiloge for x86
 and ARM
List-Id: Discussions about the Xenomai project <xenomai.xenomai.org>
List-Unsubscribe: <http://www.xenomai.org/mailman/options/xenomai>,
 <mailto:xenomai-request@xenomai.org?subject=unsubscribe>
List-Archive: <http://www.xenomai.org/pipermail/xenomai/>
List-Post: <mailto:xenomai@xenomai.org>
List-Help: <mailto:xenomai-request@xenomai.org?subject=help>
List-Subscribe: <http://www.xenomai.org/mailman/listinfo/xenomai>,
 <mailto:xenomai-request@xenomai.org?subject=subscribe>
To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Xenomai <xenomai@xenomai.org>

On Mon, Mar 02, 2015 at 06:39:22PM +0100, Jan Kiszka wrote:
> On 2015-02-27 22:24, Gilles Chanteperdrix wrote:
> > On Fri, Feb 27, 2015 at 10:21:30PM +0100, Gilles Chanteperdrix wrote:
> >> On Fri, Feb 27, 2015 at 09:37:45PM +0100, Jan Kiszka wrote:
> >>> On 2015-02-27 21:27, Gilles Chanteperdrix wrote:
> >>>> On Fri, Feb 27, 2015 at 09:12:14PM +0100, Jan Kiszka wrote:
> >>>>> Hi,
> >>>>>
> >>>>> just pushed a first implementation of the general model that I proposed
> >>>>> for exception handling. You can find it at
> >>>>>
> >>>>> http://git.xenomai.org/ipipe-jki.git/log/?h=queues/trap-rework
> >>>>
> >>>> Also, the clean way to pass virtual + physical flags is to use
> >>>> arch_mangle_bits. Using two longs (potentially 128 bits then) is
> >>>> completely useless since one of the longs simply has one significant
> >>>> bit.
> >>>
> >>> The costs of mangling is higher than using two regs for passing that
> >>> data as-is, both binary and LOC-wise (tried it). Plus the code is more
> >>> readable.
> >>
> >> That is false on ARM. On ARM gcc does not pass structs by values in
> >> registers. The values get passed on stack.
> > 
> > Sorry, misread the assembler. They are passed by registers, however
> > the registers get uselessly saved on stack, then restored to other
> > registers. 
> > 
> > struct foo {
> > 	int x;
> > 	int y;
> > };
> > 
> > int f(struct foo f)
> > {
> > 	return f.x + f.y;
> > }
> > 
> > Gives, with -Os:
> > 00000000 <f>:
> >    0:   b082            sub     sp, #8
> >    2:   ab02            add     r3, sp, #8
> >    4:   e903 0003       stmdb   r3, {r0, r1}
> >    8:   e89d 0009       ldmia.w sp, {r0, r3}
> >    c:   4418            add     r0, r3
> >    e:   b002            add     sp, #8
> >   10:   4770            bx      lr
> > 
> 
> Ouch. I missed that this sneaked in.
> 
> The complications with the existing mangle functions are that they do
> not play well with what I need for the existing
> ipipe_restore_root_nosync. I can open-code the latter (size increases),
> extend the former to alternatively return architectural flags (instead
> of boolean), or provide another wrapper to convert the virt bit into flags.
> 
> Hmm, or - and that's probably cleanest - I simply align
> ipipe_restore_root_nosync to ipipe_restore_root argument-wise. The
> latter takes "x" (stall) as boolean, the former as architectural flags.
> That's highly confusing anyway. And it seems there are no users to break
> in Xenomai, despite that it is exported to modules.

Or simply use arch_mangle_bits ?

-- 
					    Gilles.