On Mon, 29 Apr 2019 11:55:56 -0300 Eduardo Habkost <ehabk...@redhat.com> wrote:
> On Mon, Apr 29, 2019 at 08:21:06AM -0600, Alex Williamson wrote: > > On Sat, 27 Apr 2019 10:09:51 +0200 > > Paolo Bonzini <pbonz...@redhat.com> wrote: > > > > > On 27/04/19 07:29, Paolo Bonzini wrote: > > > > > > > >>> In my testing it looks like KVM advertises supporting the KVM_IRQFD > > > >>> resample feature, but vfio never gets the unmask notification, so the > > > >>> device remains with DisINTx set and no further interrupts are > > > >>> generated. Do we expect KVM's IRQFD with resampler to work in the > > > >>> split IRQ mode? We can certainly hope that "high performance" devices > > > >>> use MSI or MSI/X, but this would be quite a performance regression > > > >>> with > > > >>> split mode if our userspace bypass for INTx goes away. Thanks, > > > >> > > > >> arch/x86/kvm/lapic.c:kvm_ioapic_send_eoi() dumps to userspace before > > > >> kvm_ioapic_update_eoi() can handle the irq_ack_notifier_list via > > > >> kvm_notify_acked_gsi(), > > > > > > > > That wouldn't help because kvm_ioapic_update_eoi would not even be > > > > able to access vcpu->kvm->arch.vioapic (it's NULL). > > > > > > > > The following untested patch would signal the resamplefd in > > > > kvm_ioapic_send_eoi, > > > > before requesting the exit to userspace. However I am not sure how QEMU > > > > sets up the VFIO eventfds: if I understand correctly, when VFIO writes > > > > again to > > > > the irq eventfd, the interrupt request would not reach the userspace > > > > IOAPIC, but > > > > only the in-kernel LAPIC. That would be incorrect, and if my > > > > understanding is > > > > correct we need to trigger resampling from hw/intc/ioapic.c. > > > > > > Actually it's worse: because you're bypassing IOAPIC when raising the > > > irq, the IOAPIC's remote_irr for example will not be set. So split > > > irqchip currently must disable the intx fast path completely. > > > > > > I guess we could also reimplement irqfd and resamplefd in the userspace > > > IOAPIC, and run the listener in a separate thread (using "-object > > > iothread" on the command line and AioContext in the code). > > > > This sounds like a performance regression vs KVM irqchip any way we > > slice it. Was this change a mistake? Without KVM support, the > > universal support in QEMU kicks in, where device mmaps are disabled > > when an INTx occurs, forcing trapped access to the device, and we > > assume that the next access is in response to an interrupt and trigger > > our own internal EOI and re-enable mmaps. A timer acts as a > > catch-all. Needless to say, this is functional but not fast. It would > > be a massive performance regression for devices depending on INTx and > > previously using the KVM bypass to switch to this. INTx is largely > > considered a legacy interrupt, so non-x86 archs don't encounter it as > > often, S390 even explicitly disables INTx support. ARM and POWER > > likely just don't see a lot of these devices, but nearly all devices > > (except SR-IOV VFs) on x86 expect an INTx fallback mode and some > > drivers may run the device in INTx for compatibility. This split > > irqchip change was likely fine for "enterprise" users concerned only > > with modern high speed devices, but very much not for device assignment > > used for compatibility use cases or commodity hardware users. > > > > What's a good 4.0.1 strategy to resolve this? Re-instate KVM irqchip > > as the Q35 default? I can't see that simply switching to current QEMU > > handling is a viable option for performance? What about 4.1? We could > > certainly improve EOI support in QEMU, there's essentially no support > > currently, but it seems like an uphill battle for an iothread based > > userspace ioapic to ever compare to KVM handling? Thanks, > > irqchip=split and irqchip=kernel aren't guest ABI compatible, are > they? That would make it impossible to fix this in pc-q35-4.0 > for a 4.0.1 update. I suppose it would require a pc-q35-4.0.1 machine type :-\ Thanks, Alex
