linux-kernel - Re: [PATCH 3/3] KVM: SVM: Add irqchip_split() checks before enabling AVIC

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Message-ID: <1ab4bded-a6a6-0a10-d3ea-96b7c38b93f0@redhat.com>
Date:   Tue, 12 Sep 2017 16:22:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, joro@...tes.org
Subject: Re: [PATCH 3/3] KVM: SVM: Add irqchip_split() checks before enabling
 AVIC

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