[okl4-developer] KernelInterface syscall on ia32

[Carl van Schaik]

Hi Harry,

lock; nop

means, do an atomic nop. Ia-32 does not support this operation and thus 
the hardware raises an exception. This is like a type of undefined 
instruction and the kernel checks for the instruction sequence causing 
the fault to determine whether the KernelInterface call was invoked.

-
regards
Carl

Harry Vennik wrote:
> Hi,
>
> I wonder how the KernelInterface syscall actually works for ia32.
>
> In arch/ia32/libs/l4/include/syscall.h the KernelInterface syscall 
> stub is implemented as:
>
> INLINE void *
> L4_KernelInterface(L4_Word_t *ApiVersion,
>                    L4_Word_t *ApiFlags, L4_Word_t *KernelId)
> {
>     void *base_address;
>
>     __asm__ __volatile__("/* L4_KernelInterface() */                  
>   \n"
>                          "   lock; nop                               \n"
>                          :    /* outputs */
>                          "=a"(base_address),
>                          "=c"(*ApiVersion), "=d"(*ApiFlags), 
> "=S"(*KernelId)
>
>                          /*
>                           * no inputs
>                           */
>                          /*
>                           * no clobbers
>                           */
>         );
>
>     return base_address;
> }
>
> This looks like a 'nop' instruction is doing the work, which can't be 
> true because a 'nop' does nothing...
> So how do those output registers get their values?
>
> Btw: is there an ABI doc anywhere for OKL4 on ia32? The progmanual 
> only includes the ABI for ARM...
>
> Regards,
>
> Harry
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