Re: [PATCH 1/2] kvm tools: Respect ISR status in virtio header

[Anthony Liguori <aliguori@us.ibm.com>]

On 05/07/2011 04:30 AM, Ingo Molnar wrote:
> Hm, the ISR flag seems to be an explicit IRQ-ack mechanism, not just an
> optimization.
>
> Perhaps if the guest kernel side virtio driver expects us to do honor these
> acks and not inject double irqs when the virtio driver does not expect them?
>
> There's this code in drivers/virtio/virtio_pci.c:
>
>          /* reading the ISR has the effect of also clearing it so it's very
>           * important to save off the value. */
>          isr = ioread8(vp_dev->ioaddr + VIRTIO_PCI_ISR);
>
> Which seems to suggest that this ISR flag is more important than just a
> performance hint.
>
> Pekka: was this the patch perhaps that fixed the ping latency problem for you?
>
> Could any virtio gents on Cc: please confirm/deny this theory? :-)

When using PCI LNK interrupts, the ISR flag serves two purposes.  It 
indicates that an interrupt was raised (since the actual interrupt line 
may be shared) and it is used to acknowledge the interrupt (since PCI 
LNK lines are level triggered).

It seems like this patch is simply avoiding raising the interrupt line 
if the ISR has not been acknowledged yet.  I don't think there's a 
functional issue here but I'm surprised that it's a win.  There should 
be a very short window when the interrupt is lowered in the APIC but 
still not acknowledged in the ISR.

You should just be saving a pretty cheap system call.  I wonder if the 
system call is taking longer than it should..

Regards,

Anthony Liguori