Re: [PATCH 2/3] KVM: Interrupt mask notifiers for ioapic

[Avi Kivity <avi@redhat.com>]

Marcelo Tosatti wrote:
> On Sun, Jan 04, 2009 at 06:14:44PM +0200, Avi Kivity wrote:
>   
>> Allow clients to request notifications when the guest masks or unmasks a
>> particular irq line.  This complements irq ack notifications, as the guest
>> will not ack an irq line that is masked.
>>
>> Currently implemented for the ioapic only.
>> +
>> +struct kvm_irq_mask_notifier {
>> +	void (*func)(struct kvm_irq_mask_notifier *kimn, int masked);
>>     
>
> bool masked?
>
>   

Hey, I'm the bool fanatic!

>> @@ -128,6 +129,9 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
>>  			ioapic->redirtbl[index].bits |= (u32) val;
>>  			ioapic->redirtbl[index].fields.remote_irr = 0;
>>  		}
>> +		mask_after = ioapic->redirtbl[index].fields.mask;
>> +		if (mask_before != mask_after)
>> +			kvm_fire_mask_notifiers(ioapic->kvm, index, mask_after);
>>     
>
> This can generate "spurious acks", for example:
>
> - inject irq
> - guest masks, mask notifier cleans internal state to "acked".
> - guest writes EOI
>
> But I suppose thats fine, since PIT (for example) is getting spurious
> acks even without mask notifiers and so has to handle them.
>   

If the mask happened for a long time then the guest would expect not to 
receive those interrupts.  If it happened due to scheduling then we need 
to reinject them.

I suppose we could measure descheduled time and correct appropriately.  
There previously existed logic that limited the pending count to some 
value (10?) so there wouldn't be a stream of interrupts injected into 
the guest endlessly.

I expect in most cases, masking of the timer interrupt would not happen 
for short periods.  You'd use eflags.if or apic.tpr to mask for short 
periods.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.