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Subject: Re: [PATCH 3/3] KVM: SVM: Add irqchip_split() checks before enabling
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        Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Tue, 12 Sep 2017 16:22:40 +0200
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On 12/09/2017 15:17, Radim Krčmář wrote:
>>> Yeah, hacking TMR to get the VM exit could result in future bugs.
>>> We have to push split irqchip as the deafult in userspaces with this
>>> change.
>> Actually, I'm not quite sure about the advantages/disadvantages with split
>> irqchip, and how it would affect other cases, and why it was not used as
>> default currently.
> The main advantage of split irqchip is that we're moving code out of the
> kernel, and QEMU's irqchip currently has more features too.
> 
> I think it is not the default as the support for split irqchip is recent
> (v4.3) and has lower performance, so it is only used in cases that need
> the extra features.

One other difference is that in-kernel PIT is not supported with
split-irqchip, and the QEMU PIT lacks support for reinjecting lost
ticks.  But this should only be needed for very old guests at this point.

Paolo