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

[Ingo Molnar <mingo@elte.hu>]

* Anthony Liguori <aliguori@us.ibm.com> wrote:

> 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).

ok.

> 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 [...]

Thanks for confirming this - so i think we still do not understand the root 
cause of the ping latency and why this change fixed it ...

> [...] 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..

Well the optimization also avoids unnecessary VM exits (due to the injection, 
which interrupts a guest context immediately, even if it's running on another 
CPU), not just system calls - so it could be more expensive than a system call, 
right?

Thanks,

	Ingo