From: Joe deBlaquiere (jadb@redhat.com)
Date: Sun Jan 21 2001 - 22:29:58 EST
Sure, you can make syscalls in a system thread, you just have to make
sure you have timers, interrupts and all configured (i.e. not at the .
It would probably be more appropriate to put the code into the init
thread or even better to run it as /bin/init.
Now that I look at it a little deeper, I doesn't matter what context you
came from. If you look at the SWI entry point (below), it saves off
whatever CPSR was stuffed into the SPSR at interrupt. So if you do a SWI
from SVC mode, it saves off the SVC mode PSR.
vector_swi: sub sp, sp, #S_FRAME_SIZE
stmia sp, {r0 - r12} @ Calling r0 - r12
add r8, sp, #S_PC
stmdb r8, {sp, lr}^ @ Calling sp, lr
mov r7, r0
mrs r6, spsr <<<<<<< saves whichever PSR called
mov r5, lr
stmia r8, {r5, r6, r7} @ Save calling
PC, CPSR, OLD_R0
Looking a little father in, the ret_from_syscall routines (below) which
perform a context restore really doesn't care either.
Lret_no_check: mrs r0, cpsr @ disable IRQs
orr r0, r0, #I_BIT
msr cpsr, r0
ldr r0, [sp, #S_PSR] @ Get calling cpsr
msr spsr, r0
ldmia sp, {r0 - lr}^ @ Get calling r0
- lr
mov r0, r0
add sp, sp, #S_PC
ldr lr, [sp], #S_FRAME_SIZE - S_PC @ Get PC and
jump over PC, PSR, OLD_R0
movs pc, lr
Zhu, Yaozong wrote:
> Hi Joe,
> To put my question in another way: can system calls( swi intructions indeed ) be called by kernel code? It seems that SWI handling code are written to service USER mode calls. The init may run in USER mode. But clone system calls must be called from kernel mode code(in start_kernel) to make init run.
>
> -----Original Message-----
> From: owner-uclinux-dev@uClinux.org
> [mailto:owner-uclinux-dev@uClinux.org]On Behalf Of Joe deBlaquiere
> Sent: Sunday, January 21, 2001 11:30 PM
> To: uclinux-dev@uClinux.org
> Subject: Re: [uClinux-dev] About system call code in arm-uclinux, Aplio
> version
>
>
> I'm not entirely sure about this, but I think the answer to this is that
> the kernel_thread() call creates a thread which runs as a user mode
> process (using the clone call). Therefore init() is not being run in SVC
> mode.
>
> Zhu, Yaozong wrote:
>
>
>> Hi all,
>> It seems that start_kernel(init/main.c) runs in SVC mode. And in start_kernel, kernel_thread(init, NULL, 0) is called which does two swi's . My humble question is , when doing system calls from SVC mode, vector_swi(arch/armnommu/kernel/entry-armv.S) saves and restores USER mode registers, and _ret_from_sys_call does restore sp_SVC and spsr_SVC, but lr_SVC is lost, isn't this a problem, or I am wrong?
>>
>>
>>
>>
>> This message resent by the uclinux-dev@uclinux.org list server http://www.uClinux.org/
--
Joe deBlaquiere
Red Hat, Inc.
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