Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

Jeremy Fitzhardinge
2007-03-21T00:20:26Z

Matt Mackall wrote:
> On Tue, Mar 20, 2007 at 09:31:58AM -0700, Jeremy Fitzhardinge wrote:
>
>> Linus Torvalds wrote:
>>
>>> On Tue, 20 Mar 2007, Eric W. Biederman wrote:
>>>
>>>
>>>> If that is the case. In the normal kernel what would
>>>> the "the oops, we got an interrupt code do?"
>>>> I assume it would leave interrupts disabled when it returns?
>>>> Like we currently do with the delayed disable of normal interrupts?
>>>>
>>>>
>>> Yeah, disable interrupts, and set a flag that the fake "sti" can test, and
>>> just return without doing anything.
>>>
>>> (You may or may not also need to do extra work to Ack the hardware
>>> interrupt etc, which may be irq-controller specific. Once the CPU has
>>> accepted the interrupt, you may not be able to just leave it dangling)
>>>
>>>
>> So it would be something like:
>>
>> pda.intr_mask = 1; /* disable interrupts */
>> ...
>> pda.intr_mask = 0; /* enable interrupts */
>> if (xchg(&pda.intr_pending, 0)) /* check pending */
>> asm("sti"); /* was pending; isr left cpu interrupts masked */
>>
>
> I don't know that you need an xchg there. If you're still on the same
> CPU, it should all be nice and causal even across an interrupt handler.
> So it could be:
>
> pda.intr_mask = 0; /* intr_pending can't get set after this */
> if (unlikely(pda.intr_pending)) {
> pda.intr_pending = 0;
> asm("sti");
> }
>
> (This would actually need a C barrier, but I'll ignore that as this'd
> end up being asm...)
>
> But other interesting things could happen. If we never did a real CLI
> and we get preempted and switched to another CPU between clearing
> intr_mask and checking intr_pending, we get a little confused.
>

Could prevent preempt if pda.intr_mask is set. preemptible() is defined as:

# define preemptible() (preempt_count() == 0 && !irqs_disabled())

anyway, so that would be changed to look at the intr_mask rather than
eflags.
(I'm not sure if preemptible() is actually used to determine whether
preempt or not).

Alternatively, the intr_mask could be encoded in a bit of preempt_count...

J