[Qemu-devel] [PATCH for-4.1] q35: Revert to kernel irqchip

[Alex Williamson]

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 <address@hidden>
---
 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);