linux-kernel - Re: [PATCH v3 15/16] kvm: x86: ioapic: Lazy update IOAPIC EOI

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Message-ID: <3771e33d-365b-c214-3d40-bca67c2fa841@redhat.com>
Date:   Wed, 9 Oct 2019 11:21:41 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "graf@...zon.com" <graf@...zon.com>,
        "jschoenh@...zon.de" <jschoenh@...zon.de>,
        "karahmed@...zon.de" <karahmed@...zon.de>,
        "rimasluk@...zon.com" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v3 15/16] kvm: x86: ioapic: Lazy update IOAPIC EOI

On 13/09/19 21:01, Suthikulpanit, Suravee wrote:
>  	/*
> +	 * In case APICv accelerate EOI write and do not trap,
> +	 * in-kernel IOAPIC will not be able to receive the EOI.
> +	 * In this case, we do lazy update of the pending EOI when
> +	 * trying to set IOAPIC irq.
> +	 */
> +	if (kvm_apicv_eoi_accelerate(ioapic->kvm, edge))
> +		ioapic_lazy_update_eoi(ioapic, irq);
> +

This is okay for the RTC, and in fact I suggest that you make it work
like this even for Intel.  This will get rid of kvm_apicv_eoi_accelerate
and be nicer overall.

However, it cannot work for the in-kernel PIT, because it is currently
checking ps->irq_ack before kvm_set_irq.  Unfortunately, the in-kernel
PIT is relying on the ack notifier to timely queue the pt->worker work
item and reinject the missed tick.

Thus, you cannot enable APICv if ps->reinject is true.

Perhaps you can make kvm->arch.apicv_state a disabled counter?  Then
Hyper-V can increment it when enabled, PIT can increment it when
ps->reinject becomes true and decrement it when it becomes false;
finally, svm.c can increment it when an SVM guest is launched and
increment/decrement it around ExtINT handling?

(This conflicts with some of the suggestions I made earlier, which
implied the existence of apicv_state, but everything should if anything
become easier).

Paolo