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Date: Tue, 07 Aug 2007 13:06:30 +0200
From: Johan Borkhuis <j.borkhuis@domain.hid>
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Subject: Re: [Xenomai-help] Unexpected switch to secondary mode
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: Xenomai-help@domain.hid

Gilles,

Gilles Chanteperdrix wrote:
> Gilles Chanteperdrix wrote:
>  > On 8/3/07, Johan Borkhuis <j.borkhuis@domain.hid> wrote:
>  > > Philippe,
>  > >
>  > > (BTW: is this something that should be discussed in the Xenomai group,
>  > > or would it be better to move this discussion to the Adeos mailing list?)
>  > >
>  > > I do have question on this. We already discussed this problem earlier,
>  > > and I managed to find a very dirty work around for my application: I
>  > > added a lot of "dummy" functions to my application, which are spread
>  > > over the whole application. By calling these after the mlockall, but
>  > > before I switch to RT-mode I manage to eliminate most of the switches.
>  > 
>  > Before implementing the nocow patch, I opened /proc/self/maps and
>  > caused a fault on every writable page, even if ugly, this looks
>  > simpler than spreading dummy functions over the whole application.
>
> I posted the piece of code here:
> https://mail.gna.org/public/xenomai-help/2006-12/msg00168.html
>   
Thank you for this. However, it does not seem to work: when I execute 
this code it does not make any difference, when executed before I start 
a RT-task or even within a RT-task. The results are identical to the 
results as if no dummy functions were used. Only the pages that can be 
written are touched, but the code pages that cause the problem (afaik) 
are marked readonly, so they are not touched. I modified the linking, to 
mark the text-section as writable, but that also did not make any 
difference.
The fact that this does not work could be caused by the fact that you 
are accessing the pages as data instead of code.

Next I tried something different, more like a combination between my 
original approach and your approach: I tried to find a "return from 
subroutine" in each text-page, and if found call this location. It is 
extremely dirty, but it does seem to work quite well. Below is the code 
that I used:

==========

#define RET_CODE        0x4e800020        /* blr */

typedef int (*TestFunc)(void);

static void fault_vm(void)
{
    FILE           *maps = fopen("/proc/self/maps", "r");
    unsigned        begin, end, pagesize=getpagesize();
    char            buffer[128];
    int             rc, i;
    volatile int    tmp;
    TestFunc        testFunc;

    if (!maps) {
        perror("fopen");
        exit(EXIT_FAILURE);
    }

    while ((rc = fscanf(maps, "%x-%x",&begin, &end) == 2)) {
        fgets(buffer, 128, maps);
        for (; begin != end; begin += pagesize) {
            if(buffer[2] == 'w') {          /* Data section */
                *(volatile int *) begin = *(volatile int *) begin;
            }
            else if(buffer[1] == 'r') {     /* Text section */
                for(i = 0; i < pagesize; i += 4) {
                    testFunc = (void *)(begin + i);
                    tmp = *(volatile int *) (begin + i);
                    if(tmp == RET_CODE) {
                        testFunc();
                        continue;
                    }
                }
            }
        }
    }

    fclose(maps);
}

==========

Kind regards,
    Johan Borkhuis