* Anthony Liguori <aligu...@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
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