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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v5 00/18] IOMMU: Enable interrupt remapping for Intel IOMMU
Date: Mon, 9 May 2016 13:50:26 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

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

Of course, optimizing userspace cannot hurt either.


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