linux-kernel - Re: [PATCH v4 13/17] kvm: i8254: Deactivate APICv when using in-kernel PIT re-injection mode.

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Message-ID: <20191104231712.GD23545@rkaganb.lan>
Date:   Mon, 4 Nov 2019 23:17:16 +0000
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "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 v4 13/17] kvm: i8254: Deactivate APICv when using
 in-kernel PIT re-injection mode.

On Sat, Nov 02, 2019 at 10:57:47AM +0100, Paolo Bonzini wrote:
> On 01/11/19 23:41, Suthikulpanit, Suravee wrote:
> > +	/*
> > +	 * AMD SVM AVIC accelerates EOI write and does not trap.
> > +	 * This cause in-kernel PIT re-inject mode to fail
> > +	 * since it checks ps->irq_ack before kvm_set_irq()
> > +	 * and relies on the ack notifier to timely queue
> > +	 * the pt->worker work iterm and reinject the missed tick.
> > +	 * So, deactivate APICv when PIT is in reinject mode.
> > +	 */
> >  	if (reinject) {
> > +		kvm_request_apicv_update(kvm, false, APICV_DEACT_BIT_PIT_REINJ);
> >  		/* The initial state is preserved while ps->reinject == 0. */
> >  		kvm_pit_reset_reinject(pit);
> >  		kvm_register_irq_ack_notifier(kvm, &ps->irq_ack_notifier);
> >  		kvm_register_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
> >  	} else {
> > +		kvm_request_apicv_update(kvm, true, APICV_DEACT_BIT_PIT_REINJ);
> >  		kvm_unregister_irq_ack_notifier(kvm, &ps->irq_ack_notifier);
> >  		kvm_unregister_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
> 
> This is not too nice for Intel which does support (through the EOI exit
> mask) APICv even if PIT reinjection active.

Hmm, it's tempting to just make svm_load_eoi_exitmap() disable AVIC when
given a non-empty eoi_exit_bitmap, and enable it back on a clear
eoi_exit_bitmap.  This may remove the need to add special treatment to
PIT etc.

Thanks,
Roman.