x86 weirdness

x86 weirdness

[Erwin Rol]

Hallo,

after I had successfully "ported" barebox to about 5 different embedded 
x86 (mostly N270 and Z510 Atom boards) I ran into problems with a 
Celeron SU2300 CPU based board. After a lot of trail and error, which 
included the implementation of a BIOS based console cause the board does 
not have a RS232 port, it looked like the global variables were not what 
I expected them to be, namely zero.

So in arch/x86/boot/main_entry.c i changed to following;

/**
  * Called plainly from assembler that switches from real to flat mode
  *
  * @note The C environment isn't initialized yet
  */
void uboot_entry(void)
{
#if 0
         /* clear the BSS first */
         memset(__bss_start, 0x00, __bss_stop - __bss_start);
#else
         unsigned char* ptr;
         for (ptr = __bss_start; ptr < __bss_stop; ptr++)
                 *ptr = 0x00;
#endif


         start_barebox();
}


After doing that it works. I have not yet looked into the resulting asm 
to find the differences, but could someone have an explanation for why 
the memset doesn't work and the simple loop does work?

- Erwin







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Re: x86 weirdness

[Erwin Rol]

Hey x86-asm-experts,

On 31-7-2012 0:36, Erwin Rol wrote:
> Hallo,
>
> after I had successfully "ported" barebox to about 5 different embedded
> x86 (mostly N270 and Z510 Atom boards) I ran into problems with a
> Celeron SU2300 CPU based board. After a lot of trail and error, which
> included the implementation of a BIOS based console cause the board does
> not have a RS232 port, it looked like the global variables were not what
> I expected them to be, namely zero.
>
> So in arch/x86/boot/main_entry.c i changed to following;
>
> /**
>    * Called plainly from assembler that switches from real to flat mode
>    *
>    * @note The C environment isn't initialized yet
>    */
> void uboot_entry(void)
> {
> #if 0
>           /* clear the BSS first */
>           memset(__bss_start, 0x00, __bss_stop - __bss_start);
> #else
>           unsigned char* ptr;
>           for (ptr = __bss_start; ptr < __bss_stop; ptr++)
>                   *ptr = 0x00;
> #endif
>
>
>           start_barebox();
> }
>
>
> After doing that it works. I have not yet looked into the resulting asm
> to find the differences, but could someone have an explanation for why
> the memset doesn't work and the simple loop does work?

So when I use the memset code the asm looks like this below. As you can 
see the memset gets its 3 parameters from the stack (see comments below 
in the asm src). But when you look at the location when memset is called 
it looks like the caller tries to pass the parameters in registers. 
After looking into the Makefile I noticed the arch/x86/boot/* sources 
get compiled with -mregparm=3, but the other sources use normal calling 
convention. This means code from arch/x86/boot/* can not call any other 
barebox functions when they are not of type void f(void). The file 
arch/x86/boot/regs.c also calls memset but it seems the compiler notices 
that regs.c is 16bit code and so it inlines a 16bit memset version (I at 
least don't see a call function).

Is there any GCC or LD magic we can use to prevent error like this? 
Wouldn't it be better to _not_ change the calling convention in the 
Makefile, but mark the functions that need it with __attribute__ ?

00023e98 <in_pm32>:
    23e98:       8e d9                   mov    %ecx,%ds
    23e9a:       8e c1                   mov    %ecx,%es
    23e9c:       8e e1                   mov    %ecx,%fs
    23e9e:       8e e9                   mov    %ecx,%gs
    23ea0:       8e d1                   mov    %ecx,%ss
    23ea2:       bc c8 f8 03 00          mov    $0x3f8c8,%esp
    23ea7:       0f 00 df                ltr    %di
    23eaa:       31 c9                   xor    %ecx,%ecx
    23eac:       31 d2                   xor    %edx,%edx
    23eae:       31 db                   xor    %ebx,%ebx
    23eb0:       31 ed                   xor    %ebp,%ebp
    23eb2:       31 ff                   xor    %edi,%edi
    23eb4:       0f 00 d1                lldt   %cx
    23eb7:       e9 00 00 00 00          jmp    23ebc <uboot_entry>
00023ebc <uboot_entry>:
// calculate (3rd parameter) size and put it in %ecx
    23ebc:       b9 c8 78 03 00          mov    $0x378c8,%ecx
    23ec1:       81 e9 70 a2 02 00       sub    $0x2a270,%ecx
// clear %edx (2nd paremeter)
    23ec7:       31 d2                   xor    %edx,%edx
// start address in %eax (1st parameter)
    23ec9:       b8 70 a2 02 00          mov    $0x2a270,%eax
    23ece:       e8 e6 0b ff ff          call   14ab9 <memset>
    23ed3:       e9 4c a4 fe ff          jmp    e324 <start_barebox>

00014ab9 <memset>:
    14ab9:       55                      push   %ebp
    14aba:       89 e5                   mov    %esp,%ebp
    14abc:       53                      push   %ebx
// get parameter 1 (dst address)
    14abd:       8b 45 08                mov    0x8(%ebp),%eax	
// get parameter 2 (src byte to fill)
    14ac0:       8b 5d 0c                mov    0xc(%ebp),%ebx
// get parameter 3 (size)
    14ac3:       8b 4d 10                mov    0x10(%ebp),%ecx
    14ac6:       31 d2                   xor    %edx,%edx
    14ac8:       eb 04                   jmp    14ace <memset+0x15>
    14aca:       88 1c 10                mov    %bl,(%eax,%edx,1)
    14acd:       42                      inc    %edx
    14ace:       39 ca                   cmp    %ecx,%edx
    14ad0:       75 f8                   jne    14aca <memset+0x11>
    14ad2:       5b                      pop    %ebx
    14ad3:       5d                      pop    %ebp
    14ad4:       c3                      ret
















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Re: x86 weirdness

[Sascha Hauer]

Hi Erwin,

On Tue, Jul 31, 2012 at 12:49:29PM +0200, Erwin Rol wrote:
> Hey x86-asm-experts,

Surely you do not mean me ;)

> 
> On 31-7-2012 0:36, Erwin Rol wrote:
> 
> So when I use the memset code the asm looks like this below. As you
> can see the memset gets its 3 parameters from the stack (see
> comments below in the asm src). But when you look at the location
> when memset is called it looks like the caller tries to pass the
> parameters in registers. After looking into the Makefile I noticed
> the arch/x86/boot/* sources get compiled with -mregparm=3, but the
> other sources use normal calling convention. This means code from
> arch/x86/boot/* can not call any other barebox functions when they
> are not of type void f(void). The file arch/x86/boot/regs.c also
> calls memset but it seems the compiler notices that regs.c is 16bit
> code and so it inlines a 16bit memset version (I at least don't see
> a call function).
> 
> Is there any GCC or LD magic we can use to prevent error like this?
> Wouldn't it be better to _not_ change the calling convention in the
> Makefile, but mark the functions that need it with __attribute__ ?

Maybe it's easier and more straight forward to just use a for-loop here
instead of playing magic games with attributes and or flags? Are there
more functions affected or is it just memset?

Sascha

-- 
Pengutronix e.K.                           |                             |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |

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Re: x86 weirdness

[Erwin Rol]

Hey Sascha,

On 31-7-2012 15:13, Sascha Hauer wrote:
> Hi Erwin,
>
> On Tue, Jul 31, 2012 at 12:49:29PM +0200, Erwin Rol wrote:
>> Hey x86-asm-experts,
>
> Surely you do not mean me ;)
>
>>
>> On 31-7-2012 0:36, Erwin Rol wrote:
>>
>> So when I use the memset code the asm looks like this below. As you
>> can see the memset gets its 3 parameters from the stack (see
>> comments below in the asm src). But when you look at the location
>> when memset is called it looks like the caller tries to pass the
>> parameters in registers. After looking into the Makefile I noticed
>> the arch/x86/boot/* sources get compiled with -mregparm=3, but the
>> other sources use normal calling convention. This means code from
>> arch/x86/boot/* can not call any other barebox functions when they
>> are not of type void f(void). The file arch/x86/boot/regs.c also
>> calls memset but it seems the compiler notices that regs.c is 16bit
>> code and so it inlines a 16bit memset version (I at least don't see
>> a call function).
>>
>> Is there any GCC or LD magic we can use to prevent error like this?
>> Wouldn't it be better to _not_ change the calling convention in the
>> Makefile, but mark the functions that need it with __attribute__ ?
>
> Maybe it's easier and more straight forward to just use a for-loop here
> instead of playing magic games with attributes and or flags? Are there
> more functions affected or is it just memset?

There seem to be two memset calls, that could be problematic. Both 
should be easily replaceable with for-loops (like I already did by the 
first).

We just have to be very careful to not call any functions from outside 
the boot dir (expect for the main barebox entry point).

Don't you love the weird x86 world :-)

- Erwin

PS: I will prepare some x86 updates like bios-vga-output and GTP 
partition support that might be useful for others too.



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Re: x86 weirdness

[Jean-Christophe PLAGNIOL-VILLARD]

On 17:16 Tue 31 Jul     , Erwin Rol wrote:
> Hey Sascha,
> 
> On 31-7-2012 15:13, Sascha Hauer wrote:
> >Hi Erwin,
> >
> >On Tue, Jul 31, 2012 at 12:49:29PM +0200, Erwin Rol wrote:
> >>Hey x86-asm-experts,
> >
> >Surely you do not mean me ;)
> >
> >>
> >>On 31-7-2012 0:36, Erwin Rol wrote:
> >>
> >>So when I use the memset code the asm looks like this below. As you
> >>can see the memset gets its 3 parameters from the stack (see
> >>comments below in the asm src). But when you look at the location
> >>when memset is called it looks like the caller tries to pass the
> >>parameters in registers. After looking into the Makefile I noticed
> >>the arch/x86/boot/* sources get compiled with -mregparm=3, but the
> >>other sources use normal calling convention. This means code from
> >>arch/x86/boot/* can not call any other barebox functions when they
> >>are not of type void f(void). The file arch/x86/boot/regs.c also
> >>calls memset but it seems the compiler notices that regs.c is 16bit
> >>code and so it inlines a 16bit memset version (I at least don't see
> >>a call function).
> >>
> >>Is there any GCC or LD magic we can use to prevent error like this?
> >>Wouldn't it be better to _not_ change the calling convention in the
> >>Makefile, but mark the functions that need it with __attribute__ ?
> >
> >Maybe it's easier and more straight forward to just use a for-loop here
> >instead of playing magic games with attributes and or flags? Are there
> >more functions affected or is it just memset?
> 
> There seem to be two memset calls, that could be problematic. Both
> should be easily replaceable with for-loops (like I already did by
> the first).
> 
> We just have to be very careful to not call any functions from
> outside the boot dir (expect for the main barebox entry point).
> 
> Don't you love the weird x86 world :-)
so I guess we should update the x86 to use the pbl so we can have two explicit
part of binary and we can avoid those issue

Best Regards,
J.

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Re: x86 weirdness

[Erwin Rol]

On 31-7-2012 17:51, Jean-Christophe PLAGNIOL-VILLARD wrote:
> On 17:16 Tue 31 Jul     , Erwin Rol wrote:
>> Hey Sascha,
>>
>> On 31-7-2012 15:13, Sascha Hauer wrote:
>>> Hi Erwin,
>>>
>>> On Tue, Jul 31, 2012 at 12:49:29PM +0200, Erwin Rol wrote:
>>>> Hey x86-asm-experts,
>>>
>>> Surely you do not mean me ;)
>>>
>>>>
>>>> On 31-7-2012 0:36, Erwin Rol wrote:
>>>>
>>>> So when I use the memset code the asm looks like this below. As you
>>>> can see the memset gets its 3 parameters from the stack (see
>>>> comments below in the asm src). But when you look at the location
>>>> when memset is called it looks like the caller tries to pass the
>>>> parameters in registers. After looking into the Makefile I noticed
>>>> the arch/x86/boot/* sources get compiled with -mregparm=3, but the
>>>> other sources use normal calling convention. This means code from
>>>> arch/x86/boot/* can not call any other barebox functions when they
>>>> are not of type void f(void). The file arch/x86/boot/regs.c also
>>>> calls memset but it seems the compiler notices that regs.c is 16bit
>>>> code and so it inlines a 16bit memset version (I at least don't see
>>>> a call function).
>>>>
>>>> Is there any GCC or LD magic we can use to prevent error like this?
>>>> Wouldn't it be better to _not_ change the calling convention in the
>>>> Makefile, but mark the functions that need it with __attribute__ ?
>>>
>>> Maybe it's easier and more straight forward to just use a for-loop here
>>> instead of playing magic games with attributes and or flags? Are there
>>> more functions affected or is it just memset?
>>
>> There seem to be two memset calls, that could be problematic. Both
>> should be easily replaceable with for-loops (like I already did by
>> the first).
>>
>> We just have to be very careful to not call any functions from
>> outside the boot dir (expect for the main barebox entry point).
>>
>> Don't you love the weird x86 world :-)
> so I guess we should update the x86 to use the pbl so we can have two explicit
> part of binary and we can avoid those issue

One thing to know about the x86 support is that it uses (16bit) bios 
calls to load data from disk and print info on the (vga) screen. These 
calls are wrapped with special code to change from 32bit to 16bit and 
back. So I have no idea of "pbl" works with this very special x86 code.

- Erwin


> Best Regards,
> J.
>


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Re: x86 weirdness

[Sascha Hauer]

On Tue, Jul 31, 2012 at 05:16:26PM +0200, Erwin Rol wrote:
> Hey Sascha,
> 
> 
> There seem to be two memset calls, that could be problematic. Both
> should be easily replaceable with for-loops (like I already did by
> the first).
> 
> We just have to be very careful to not call any functions from
> outside the boot dir (expect for the main barebox entry point).

This is probably a good idea anyway since the bss is not cleared at this
stage.

> 
> Don't you love the weird x86 world :-)

I'm glad that most of the time I don't have to care about x86 ;)

Sascha

-- 
Pengutronix e.K.                           |                             |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |

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