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Re: [Qemu-devel] [PATCH] ARM: Fix disable interrupt for M profile

From: Fabien Chouteau
Subject: Re: [Qemu-devel] [PATCH] ARM: Fix disable interrupt for M profile
Date: Fri, 31 May 2013 11:04:37 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130329 Thunderbird/17.0.5

On 05/30/2013 07:26 PM, Peter Maydell wrote:
> On 30 May 2013 17:22, Fabien Chouteau <address@hidden> wrote:
>> I'm not sure this was expected or not, but it looks like the "||" should
>> be a "&&". Otherwise it's not possible to disable interrupt.
>> Signed-off-by: Fabien Chouteau <address@hidden>
> We've had people trying to fiddle with this bit of code
> before. I'd like to see a clear description of:
>  * the bug (ideally with a test case)

The bug: It's impossible to disable interrupt on M-profile.
Test case: I can try to build a binary, the source will be in Ada.

>  * why this patch fixes it (probably with reference to the
>    architecture manual and to what QEMU means when it sets
>    "CPSR_I" on M profile [hint: look at how we handle PRIMASK])

I do not intend to do a full review of interrupt handling in the
M-profile. I just noticed that disabling interrupt was not effective,
and found this condition that looked faulty to me. I want to fix the
condition to match requirements of the comment above.

>  * a demonstration that it doesn't break non-M-profile

Much more difficult for me, but that's what peer-review is for, right?.

>> @@ -462,8 +462,8 @@ int cpu_exec(CPUArchState *env)
>>                         We avoid this by disabling interrupts when
>>                         pc contains a magic address.  */
>>                      if (interrupt_request & CPU_INTERRUPT_HARD
>> -                        && ((IS_M(env) & env->regs[15] < 0xfffffff0)
>> -                            || !(env->uncached_cpsr & CPSR_I))) {
>> +                        && (IS_M(env) && env->regs[15] < 0xfffffff0)
>> +                        && !(env->uncached_cpsr & CPSR_I)) {
>>                          env->exception_index = EXCP_IRQ;
>>                          cc->do_interrupt(cpu);
>>                          next_tb = 0;
> ...for instance this change means that the if() condition
> will now never be satisfied for a non-M-profile core,
> which is definitely wrong.

You're right I did this a little bit too quickly.

The expression should be:

    if (interrupt_request & CPU_INTERRUPT_HARD
        && (!IS_M(env) || env->regs[15] < 0xfffffff0)
        && !(env->uncached_cpsr & CPSR_I)) {

I'll resend a patch, just in case.


Fabien Chouteau

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