On Fri, 26 Apr 2019 13:27:44 -0600 Alex Williamson <alex.william...@redhat.com> wrote:
> On Thu, 20 Dec 2018 13:40:35 +0800 > Peter Xu <pet...@redhat.com> wrote: > > > Starting from QEMU 4.0, let's specify "split" as the default value for > > kernel-irqchip. > > > > So for QEMU>=4.0 we'll have: allowed=Y,required=N,split=Y > > for QEMU<=3.1 we'll have: allowed=Y,required=N,split=N > > (omitting all the "kernel_irqchip_" prefix) > > > > Note that this will let the default q35 machine type to depend on > > Linux version 4.4 or newer because that's where split irqchip is > > introduced in kernel. But it's fine since we're boosting supported > > Linux version for QEMU 4.0 to around Linux 4.5. For more information > > please refer to the discussion on AMD's RDTSCP: > > > > https://lore.kernel.org/lkml/20181210181328.ga...@zn.tnic/ > > It looks like this broke INTx for vfio-pci, see: > > https://bugs.launchpad.net/qemu/+bug/1826422 > > 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(), so it looks like KVM really ought to return an error when trying to register a resample IRQFD when irqchip_split(), but do we have better options? EOI handling in QEMU is pretty much non-existent and even if it was in place, it's a big performance regression for vfio INTx handling. Is a split irqchip that retains performant resampling (EOI) support possible? Thanks, Alex
