Re: [Xen-devel] [PATCH ARM v6 09/14] mini-os: arm: scheduling

[Thomas Leonard <talex5@xxxxxxxxx>]

On 28 July 2014 11:53, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Wed, 2014-07-16 at 12:07 +0100, Thomas Leonard wrote:
>> Based on an initial patch by Karim Raslan.
>>
>> Signed-off-by: Karim Allah Ahmed <karim.allah.ahmed@xxxxxxxxx>
>> Signed-off-by: Thomas Leonard <talex5@xxxxxxxxx>
>> ---
>>  extras/mini-os/arch/arm/arm32.S | 22 ++++++++++++++++++++++
>>  extras/mini-os/arch/arm/sched.c | 38 ++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 60 insertions(+)
>>  create mode 100644 extras/mini-os/arch/arm/sched.c
>>
>> diff --git a/extras/mini-os/arch/arm/arm32.S 
>> b/extras/mini-os/arch/arm/arm32.S
>> index 88efe24..56429b1 100644
>> --- a/extras/mini-os/arch/arm/arm32.S
>> +++ b/extras/mini-os/arch/arm/arm32.S
>> @@ -204,8 +204,30 @@ irq_handler:
>>  IRQ_handler:
>>       .long   0x0
>>
>> +
>> +.globl __arch_switch_threads
>> +@ => r0 = &prev->sp
>> +@    r1 = &next->sp
>> +@ <= returns to next thread's saved return address
>> +__arch_switch_threads:
>> +     stmia   r0, {sp, lr}    @ Store current sp and ip to prev's struct 
>> thread
>> +     str     fp, [sp, #-4]   @ Store fp on the old stack
>> +
>> +     ldmia   r1, {sp, lr}    @ Load new sp, ip from next's struct thread
>> +     ldr     fp, [sp, #-4]   @ Restore fp from the stack
>> +
>> +     mov     pc, lr
>
> I'm a bit confused by the lack of saving of the GPRs here. Shouldn't at
> least the caller saved registers (r4..r11) be saved/restored here?

If they were caller saved then sched.c would save them itself before
calling switch_threads. But the AAPCS says:

"A subroutine must preserve the contents of the registers r4-r8, r10,
r11 and SP (and r9 in PCS variants that designate r9 as v6)."

So I think these are callee-saved and we should save them here.

>> +void run_idle_thread(void)
>> +{
>> +    __asm__ __volatile__ ("mov sp, %0; mov pc, %1"::"r"(idle_thread->sp), 
>> "r"(idle_thread->ip));
>
> Is bx preferred to mov pc these days? I don't know if it matters in this
> case...

Yes, that would probably be better (in both places).


-- 
Dr Thomas Leonard        http://0install.net/
GPG: 9242 9807 C985 3C07 44A6  8B9A AE07 8280 59A5 3CC1
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