Re: [PATCH] KVM: avoid taking ioapic mutex for non-ioapic EOIs

[Avi Kivity <avi@redhat.com>]

On 12/28/2009 10:37 PM, Marcelo Tosatti wrote:
> On Mon, Dec 28, 2009 at 02:08:30PM +0200, Avi Kivity wrote:
>    
>> When the guest acknowledges an interrupt, it sends an EOI message to the local
>> apic, which broadcasts it to the ioapic.  To handle the EOI, we need to take
>> the ioapic mutex.
>>
>> On large guests, this causes a lot of contention on this mutex.  Since large
>> guests usually don't route interrupts via the ioapic (they use msi instead),
>> this is completely unnecessary.
>>
>> Avoid taking the mutex by introducing a handled_vectors bitmap.  Before taking
>> the mutex, check if the ioapic was actually responsible for the acked vector.
>> If not, we can return early.
>>      
> Can't you skip IOAPIC EOI for edge triggered interrupts (in the LAPIC
> code), instead?
>    

That's a lot cleaner, yes.  Indeed there's the TMR which holds this 
info.  Gleb suggested doing this in the local apic but we didn't think 
of using the TMR.

There's a small race there - the TMR is set after the IRR, so the 
interrupt can be injected and acked before the TMR is updated, but that 
can be fixed by switching the order.

But what about kvm_notify_acked_irq() in __kvm_ioapic_update_eoi()?

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