[Qemu-devel] [PATCH for-4.0.1] 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 with a
new pc-q35-4.0.1 machine type for qemu-stable while the development
branch makes the same change 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.  This new 4.0.1 machine type
makes this change automatically.

Link: https://bugs.launchpad.net/qemu/+bug/1826422
Link: https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg03305.html
Fixes: b2fc91db8447 ("q35: set split kernel irqchip as default")
Cc: address@hidden
Signed-off-by: Alex Williamson <address@hidden>
---

Do we want new stable versions for other archs too or only as needed?

 hw/core/machine.c    |    3 +++
 hw/i386/pc.c         |    3 +++
 hw/i386/pc_q35.c     |   16 ++++++++++++++--
 include/hw/boards.h  |    3 +++
 include/hw/i386/pc.h |    3 +++
 5 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 743fef28982c..5d046a43e3d2 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -24,6 +24,9 @@
 #include "hw/pci/pci.h"
 #include "hw/mem/nvdimm.h"
 
+GlobalProperty hw_compat_4_0[] = {};
+const size_t hw_compat_4_0_len = G_N_ELEMENTS(hw_compat_4_0);
+
 GlobalProperty hw_compat_3_1[] = {
     { "pcie-root-port", "x-speed", "2_5" },
     { "pcie-root-port", "x-width", "1" },
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index f2c15bf1f2c3..d98b737b8f3b 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -115,6 +115,9 @@ struct hpet_fw_config hpet_cfg = {.count = UINT8_MAX};
 /* Physical Address of PVH entry point read from kernel ELF NOTE */
 static size_t pvh_start_addr;
 
+GlobalProperty pc_compat_4_0[] = {};
+const size_t pc_compat_4_0_len = G_N_ELEMENTS(pc_compat_4_0);
+
 GlobalProperty pc_compat_3_1[] = {
     { "intel-iommu", "dma-drain", "off" },
     { "Opteron_G3" "-" TYPE_X86_CPU, "rdtscp", "off" },
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 372c6b73bebd..45cc29d1adb7 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);
@@ -365,12 +365,24 @@ static void pc_q35_machine_options(MachineClass *m)
     m->max_cpus = 288;
 }
 
-static void pc_q35_4_0_machine_options(MachineClass *m)
+static void pc_q35_4_0_1_machine_options(MachineClass *m)
 {
     pc_q35_machine_options(m);
     m->alias = "q35";
 }
 
+DEFINE_Q35_MACHINE(v4_0_1, "pc-q35-4.0.1", NULL,
+                   pc_q35_4_0_1_machine_options);
+
+static void pc_q35_4_0_machine_options(MachineClass *m)
+{
+    pc_q35_4_0_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);
+}
+
 DEFINE_Q35_MACHINE(v4_0, "pc-q35-4.0", NULL,
                    pc_q35_4_0_machine_options);
 
diff --git a/include/hw/boards.h b/include/hw/boards.h
index e231860666a1..fe1885cbffa0 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -293,6 +293,9 @@ struct MachineState {
     } \
     type_init(machine_initfn##_register_types)
 
+extern GlobalProperty hw_compat_4_0[];
+extern const size_t hw_compat_4_0_len;
+
 extern GlobalProperty hw_compat_3_1[];
 extern const size_t hw_compat_3_1_len;
 
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index ca65ef18afb4..43df7230a22b 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -293,6 +293,9 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
 int e820_get_num_entries(void);
 bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *);
 
+extern GlobalProperty pc_compat_4_0[];
+extern const size_t pc_compat_4_0_len;
+
 extern GlobalProperty pc_compat_3_1[];
 extern const size_t pc_compat_3_1_len;