On Mon, Nov 11, 2019 at 11:37:53AM -0600, Suravee Suthikulpanit wrote: > 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. This doesn't look like a hot path, so I'm not sure it needs to be optimized for performance. Especially so since kvm_make_scan_ioapic_request does kvm_make_all_cpus_request which isn't particularly fast by definition, and I guess the extra overhead there won't be noticable. OTOH introducing extra code paths has its maintenance costs, so sticking the simple logic in svm_load_eoi_exitmap looks attractive. Just my 2c, Roman.
