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

[Roman Kagan]

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.