RFC: use EM_LEGACY_IRQ in if_lem.c ?

[Adrian Chadd]

On 25 July 2012 08:14, Luigi Rizzo <rizzo at iet.unipi.it> wrote:

>> I suggest doing some digging to understand why. I bet we all know the
>> answer, but it would be nice to have it documented and investigated. I
>> bet em(4) isn't the only device that would benefit from this?
>
> I am not so sure i know the answer on bare iron (and my take is that the
> difference is more or less irrelevant there), but in the virtualized case
> the improvement is almost surely because the code used in FAST_INTR
> has a couple of MMIO accesses to disable/enable interrupts on the
> card while the taskqueue runs.  These are expensive in a VM
> (such accesses cost ~10K cycles each, even with hw support)

Hm, really? Doing these register accesses to a virtualised em NIC in a
VM is that expensive, or is there something else going on I don't
understand?



Adrian