On Tue, 14 May 2019 12:46:47 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:
> Commit b2fc91db8447 ("q35: set split kernel irqchip as default") changed
> the default for the pc-q35-4.0 machine type to use split irqchip, which
> turned out to have disasterous effects on vfio-pci INTx support. KVM
> resampling irqfds are registered for handling these interrupts, but
> these are non-functional in split irqchip mode. We can't simply test
> for split irqchip in QEMU as userspace handling of this interrupt is a
> significant performance regression versus KVM handling (GeForce GPUs
> assigned to Windows VMs are non-functional without forcing MSI mode or
> re-enabling kernel irqchip).
>
> The resolution is to revert the change in default irqchip mode in the
> pc-q35-4.1 machine type. The qemu-q35-4.0 machine type should not be
> used in vfio-pci configurations for devices requiring legacy INTx
> support without explicitly modifying the VM configuration to use KVM
> irqchip. A new pc-q35-4.0.1 machine type is submitted to resolve this
> in the stable branch.
>
> Link: https://bugs.launchpad.net/qemu/+bug/1826422
> Fixes: b2fc91db8447 ("q35: set split kernel irqchip as default")
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
> hw/i386/pc_q35.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> index 37dd350511a9..9f90dc72a53f 100644
> --- a/hw/i386/pc_q35.c
> +++ b/hw/i386/pc_q35.c
> @@ -357,7 +357,7 @@ static void pc_q35_machine_options(MachineClass *m)
> m->units_per_default_bus = 1;
> m->default_machine_opts = "firmware=bios-256k.bin";
> m->default_display = "std";
> - m->default_kernel_irqchip_split = true;
> + m->default_kernel_irqchip_split = false;
> m->no_floppy = 1;
> machine_class_allow_dynamic_sysbus_dev(m, TYPE_AMD_IOMMU_DEVICE);
> machine_class_allow_dynamic_sysbus_dev(m, TYPE_INTEL_IOMMU_DEVICE);
> @@ -377,6 +377,7 @@ DEFINE_Q35_MACHINE(v4_1, "pc-q35-4.1", NULL,
> static void pc_q35_4_0_machine_options(MachineClass *m)
> {
> pc_q35_4_1_machine_options(m);
> + m->default_kernel_irqchip_split = true;
> m->alias = NULL;
> compat_props_add(m->compat_props, hw_compat_4_0, hw_compat_4_0_len);
> compat_props_add(m->compat_props, pc_compat_4_0, pc_compat_4_0_len);
>
Hmm, I need to make 4.0.1 here too, v2 coming...