Re: [Xen-devel] [PATCH 2/5] xen/arm: Keep count of inflight interrupts

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-06-26 at 12:50 +0100, Stefano Stabellini wrote:
> On Wed, 26 Jun 2013, Ian Campbell wrote:

> > In that case my comments about deprioritisation and actual EOI happening
> > later are correct, aren't they?
> 
> No, they are not. This is the full sequence of events, including
> deprioritization and EOI:
> 
> - the vtimer interrupt fires
> - xen deprioritizes the vtimer interrupt
> - xen masks the vtimer interrupt
> - xen adds the vtimer interrupt to the LR for the guest
> - xen EOIs the vtimer interrupt
> - the guest receives the vtimer interrupt
> - the guest deprioritizes the vtimer interrupt
> - the guest set the next deadline in the past
> - the guest enables the vtimer
> ## an unexpected vtimer interrupt is received by Xen but the current
> ## one is still being serviced 
> - the guest EOIs the vtimer interrupt

Is this particular to the handling of the vtimer interrupt? In the
normal case an interrupt which is injected into the guest isn't EOIed by
Xen until the maintenance interrupt, isn't it? But the vtimer is special
because it isn't routed to the guest even though we do so via an
orthogonal mechanism?

Perhaps the solution here is a third type of interrupt alongside
"for-xen" and "for-a-specific-guest" which is "for-current-guest"?

Ian.


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