Re: [Xen-devel] schedule() vs softirqs

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On Fri, 2006-12-15 at 17:36 +0000, Keir Fraser wrote:
> On 15/12/06 17:27, "Hollis Blanchard" <hollisb@xxxxxxxxxx> wrote:
> 
> > We recently uncovered a bug on PowerPC where if a timer tick arrives
> > just inside schedule() while interrupts are still enabled, the
> > decrementer is never reprogrammed to that appropriate value. This is
> > because once inside schedule(), we never handle any subsequent softirqs:
> > we call context_switch() and resume the guest.
> 
> Easily fixed. You need to handle softirqs in the exit path to guest context.
> You need to do this final check with interrupts disabled to avoid races.

Ah OK, I see now how x86 is doing that. I don't think that code flow
really makes sense: why would you jump out of do_softirq() into assembly
just to call do_softirq() again?

Also, that doesn't solve the lazy register saving problem.

However, I think I see how we can implement our desired context_switch()
scheme in arch-specific code. The context_switch() call in schedule()
will return, so please don't add a BUG() after that. :)

-- 
Hollis Blanchard
IBM Linux Technology Center


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