Re: [PATCH 3/3] kvm-s390: streamline memslot handling

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Am Dienstag 26 Mai 2009 09:57:58 schrieb Avi Kivity:
> > I could add that behaviour, but that could make our normal interrupt
> > handling much slower. Therefore I don't want to call that function,
> > but on the other hand I like the "skip if the request is already set"
> > functionality and think about adding that in my loop.
>
> I don't understand why it would affect your interrupt handling.  We need

As far as I understand x86, every host interrupt causes a guest exit.

On s390 the SIE instruction is interruptible. On a host interrupt (like an 
IPI)  the host interrupt handler runs and finally jumps back into the SIE 
instruction.  The hardware will continue with guest execution. This has the 
advantage, that we dont have to load/save guest and host registers on host 
interrupts. (the low level interrupt handler saves the registers of the 
interrupted context)

In our low-level interrupt handler we do check for signal_pending, 
machine_check_pending and need_resched to leave the sie instruction. For 
anything else a the host sees a cpu bound guest always in the SIE instruction. 

Christian


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