On 03.01.2014, at 03:51, Scott Wood <scottwood@xxxxxxxxxxxxx> wrote:
> On Sun, 2013-12-29 at 16:43 +0100, Alexander Graf wrote:
>> On 11.07.2013, at 00:47, Scott Wood <scottwood@xxxxxxxxxxxxx> wrote:
>>> diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
>>> index ddfaf56..c13caa0 100644
>>> --- a/arch/powerpc/kvm/book3s_pr.c
>>> +++ b/arch/powerpc/kvm/book3s_pr.c
>>> @@ -884,14 +884,11 @@ program_interrupt:
>>> * and if we really did time things so badly, then we just exit
>>> * again due to a host external interrupt.
>>> */
>>> - local_irq_disable();
>>> s = kvmppc_prepare_to_enter(vcpu);
>>> - if (s <= 0) {
>>> - local_irq_enable();
>>> + if (s <= 0)
>>> r = s;
>>> - } else {
>>> + else
>>> kvmppc_fix_ee_before_entry();
>>> - }
>>> }
>>
>> Please add a comment here stating that interrupts are hard disabled at this point.
>
> OK.
>
>>> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
>>> index 4e05f8c..2f7a221 100644
>>> --- a/arch/powerpc/kvm/powerpc.c
>>> +++ b/arch/powerpc/kvm/powerpc.c
>>> @@ -64,12 +64,14 @@ int kvmppc_prepare_to_enter(struct kvm_vcpu *vcpu)
>>> {
>>> int r = 1;
>>>
>>> - WARN_ON_ONCE(!irqs_disabled());
>>> + WARN_ON(irqs_disabled());
>>> + hard_irq_disable();
>>> +
>>> while (true) {
>>> if (need_resched()) {
>>> local_irq_enable();
>>
>> One thing I find reasonably tricky in this approach is that we run
>> local_irq_enable, but hard_irq_disable. I did dig through the code and
>> it should work just fine, but I think we should make sure to note that
>> this is intended and doesn't just work by accident.
>>
>> Just add a comment above the first call to hard_irq_disable() that
>> explains that local_irq_enable() will properly unroll hard_irq_disable.
>> That way the next person reading this doesn't have to dig as deeply.
>
> There is no hard_irq_enable() -- only __hard_irq_enable() that doesn't
> update local_paca->irq_happened.
>
> This is normal use of the API. If there does need to be a comment, it
> should go in hw_irq.h. :-)
Yeah, it's always confusing to me in other places too :). But there are only so many places that have to deal with really hard disabled interrupts.
>
>>> #ifdef CONFIG_PPC64
>>> - /* lazy EE magic */
>>> - hard_irq_disable();
>>> - if (lazy_irq_pending()) {
>>> - /* Got an interrupt in between, try again */
>>> - local_irq_enable();
>>> - local_irq_disable();
>>> - kvm_guest_exit();
>>> - continue;
>>> - }
>>> + WARN_ON(lazy_irq_pending());
>>> #endif
>>> + /* Can't use irqs_disabled() because we want hard irq state */
>>> + WARN_ON(mfmsr() & MSR_EE);
>>
>> The reason for lazy EE is that mfmsr() is supposed to be slow.
>
> Not just mtmsr?
>
> And when I complained about wrtee not being all that slow on our
> hardware, Ben said it was also for perf coverage. :-)
>
>> Couldn't we check for the internal hard disable flag instead? Just create a new
>> helper in hw_irq.h that tells us
>>
>> local_paca->irq_happened & PACA_IRQ_HARD_DIS
>>
>> on PPC64 and
>>
>> !(mfmsr() & MSR_EE)
>>
>> on PPC32. We can then just use that helper here and not run "expensive" mfmsr() calls.
>
>> I'm not sure whether it's actually measurable overhead in the context
>> of the whole world switch, but why diverge from the rest of the system?
>> If you think a new helper is overkill, I'd be fine with a simple #ifdef
>> here just as well.
>
> I'd hope a mfmsr wouldn't be so slow on any hardware as to be a big deal
> here, though it'd also be nice if there were a Linux-wide way of
> specifying whether a particular WARN should be always checked, or only
> if the kernel is built with some debug option...
>
> The "rest of the system" is irqs_disabled() and there's already a
> comment explaining why we diverge from that.
>
> Maybe we should just remove that check; we'd still at least have the
> 64-bit check in kvmppc_fix_ee_before_entry.
Well, as I mentioned we're already so deep down in the guts of how interrupt handling works that it's perfectly fine to check paca->irq_happened directly here if we care.
>
>>> kvm_guest_enter();
>>> - break;
>>> + return r;
>>
>> This must be 0 at this point, right?
>
> No, it'll be 1.
>
>> Either way, I prefer to keep the code easily understandable. How about
>> you keep the break and just add an if (r) around the local_irq_enable()
>> below? Then add a comment stating that the return value tells us
>> whether entry is ok to proceed and interrupts are disabled (0) or
>> something didn't work out and interrupts are enabled (!0).
>
> How is it more understandable to overload what looks like a single code
> path with divergent cases that have little to nothing in common? Would
> it help to add a comment on the return-to-host code path indicating that
> it is only used for returning to the host?
Yup :)
> I'd be fine with replacing "return r" with "return 1" (and getting rid
> of the initialization of r at the top of the function, unless GCC
> complains inappropriately).
Works fine for me.
Alex
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