Re: [Qemu-devel] [PATCH v5 00/18] IOMMU: Enable interrupt remapping for Intel IOMMU

[Paolo Bonzini]

On 03/05/2016 06:38, Jan Kiszka wrote:
> Yes, this doesn't look good from the performance POV. Given that most
> EOIs of the APICs will not trigger a message to an IOAPIC and userspace
> exits are expensive, that should be measurable.
> 
> But you should optimize route updating a bit: I noticed real delays
> (almost a second) when reprogramming all IOAPIC pins during Jailhouse
> handover. That's because of the heavyweight synchronize_srcu we have on
> each route change. Even if that is an extreme case, you should try to
> reduce route updates to only those that were actually affected by an
> IRTE change.

It's okay to make it synchronize_srcu_expedited.  Unlike RCU, expediting
SRCU grace periods doesn't have an impact on the whole system (the
reason why KVM uses SRCU here is exactly to avoid
synchronize_rcu_expedited).

Of course, optimizing userspace cannot hurt either.

Paolo