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

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Date:   Mon, 11 Nov 2019 11:37:53 -0600
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "rkagan@...tuozzo.com" <rkagan@...tuozzo.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.

Roman/Paolo

On 11/5/2019 4:47 PM, Paolo Bonzini wrote:
> On 05/11/19 00:17, Roman Kagan wrote:
>>> 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.
> 
> That is a very nice idea---we can make that a single disable reason,
> like APICV_DEACTIVATE_REASON_EOI, and Intel can simply never use it.

I took at look at the svm_load_eoi_exitmap() and it is called via:
     kvm_make_scan_ioapic_request() ->
         KVM_REQ_SCAN_IOAPIC -> vcpu_scan_ioapic() ->
             KVM_REQ_LOAD_EOI_EXITMAP -> vcpu_load_eoi_exitmap()

The kvm_make_scan_ioapic_request() is called from multiple places:

arch/x86/kvm/irq_comm.c:
     * kvm_arch_post_irq_routing_update() : Called from kvm_set_irq_routing()

arch/x86/kvm/ioapic.c:
     * kvm_arch_post_irq_ack_notifier_list_update() : (Un)registering irq ack notifier
     * kvm_set_ioapic() : Setting ioapic irqchip
     * ioapic_mmio_write() -> ioapic_write_indirect()

arch/x86/kvm/lapic.c:
     * recalculate_apic_map()

Most calls would be from ioapic_mmio_write()->ioapic_write_indirect().

In case of AMD AVIC, the svm_load_e::vsoi_exitmap() is called several times, and requesting
APICV (de)activate from here when the eoi_exit_bitmap is set/clear would introduce
large overhead especially with SMP machine. So, for now, let's just disable APICv
when in-kernel PIT is in reinject (delay) mode.

I'll also add the logic to avoid unnecessary overhead for Intel.

Thanks,
Suravee