Re: [Qemu-devel] [PATCH for-4.1 v2] q35: Revert to kernel irqchip

[Paolo Bonzini]

On 29/05/19 05:30, Michael S. Tsirkin wrote:
> On Tue, May 14, 2019 at 02:14:41PM -0600, Alex Williamson 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 and create a pc-q35-4.0.1 machine for the 4.0-stable
>> branch.  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 kernel irqchip.
>>
>> Link: https://bugs.launchpad.net/qemu/+bug/1826422
>> Fixes: b2fc91db8447 ("q35: set split kernel irqchip as default")
>> Signed-off-by: Alex Williamson <address@hidden>
> 
> OK I guess but it's really a kvm patch.
> So I'd like Paolo to review and merge if appropriate.
> 
> Can't say this makes me too happy. split irqchip
> has a bunch of advantages.

Yeah, me too but I don't see an alternative.  I'll merge it today.

Paolo

> 
>> ---
>>  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 5d046a43e3d2..e41e6698ac9f 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_1[] = {};
>> +const size_t hw_compat_4_0_1_len = G_N_ELEMENTS(hw_compat_4_0_1);
>> +
>>  GlobalProperty hw_compat_4_0[] = {};
>>  const size_t hw_compat_4_0_len = G_N_ELEMENTS(hw_compat_4_0);
>>  
>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>> index d98b737b8f3b..b5311e7e2bd5 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_1[] = {};
>> +const size_t pc_compat_4_0_1_len = G_N_ELEMENTS(pc_compat_4_0_1);
>> +
>>  GlobalProperty pc_compat_4_0[] = {};
>>  const size_t pc_compat_4_0_len = G_N_ELEMENTS(pc_compat_4_0);
>>  
>> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
>> index 37dd350511a9..dcddc6466200 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);
>> @@ -374,10 +374,22 @@ static void pc_q35_4_1_machine_options(MachineClass *m)
>>  DEFINE_Q35_MACHINE(v4_1, "pc-q35-4.1", NULL,
>>                     pc_q35_4_1_machine_options);
>>  
>> -static void pc_q35_4_0_machine_options(MachineClass *m)
>> +static void pc_q35_4_0_1_machine_options(MachineClass *m)
>>  {
>>      pc_q35_4_1_machine_options(m);
>>      m->alias = NULL;
>> +    compat_props_add(m->compat_props, hw_compat_4_0_1, hw_compat_4_0_1_len);
>> +    compat_props_add(m->compat_props, pc_compat_4_0_1, pc_compat_4_0_1_len);
>> +}
>> +
>> +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);
>>  }
>> diff --git a/include/hw/boards.h b/include/hw/boards.h
>> index 6f7916f88f02..6ff02bf3e472 100644
>> --- a/include/hw/boards.h
>> +++ b/include/hw/boards.h
>> @@ -292,6 +292,9 @@ struct MachineState {
>>      } \
>>      type_init(machine_initfn##_register_types)
>>  
>> +extern GlobalProperty hw_compat_4_0_1[];
>> +extern const size_t hw_compat_4_0_1_len;
>> +
>>  extern GlobalProperty hw_compat_4_0[];
>>  extern const size_t hw_compat_4_0_len;
>>  
>> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
>> index 43df7230a22b..5d5636241e34 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_1[];
>> +extern const size_t pc_compat_4_0_1_len;
>> +
>>  extern GlobalProperty pc_compat_4_0[];
>>  extern const size_t pc_compat_4_0_len;
>>