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

["Michael S. Tsirkin" <mst@redhat.com>]

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 <alex.williamson@redhat.com>

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.

> ---
>  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;
>