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

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

On Wed, May 29, 2019 at 07:43:56AM -0600, Alex Williamson wrote:
> On Tue, 28 May 2019 23:30:20 -0400
> "Michael S. Tsirkin" <mst@redhat.com> 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 <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.
> 
> What exactly are those advantages relative to a our default QEMU
> users?  I see lots of users assigning consumer class hardware and
> seeing regressions.  They don't care about a theoretically improved
> security attack surface at the cost of performance for legacy devices,
> or especially functionality.  Should these users be our focus for the
> default machine type?  I think we could make these sorts of compromises
> in a legacy-free or virt/cloud/enterprise machine type, but for a
> general purpose PC emulator, it doesn't seem a good match given the
> issues we have currently.  Obviously users can continue to tune the q35
> machine type on their own and enable split irqchip if it's important to
> them.  Thanks,
> 
> Alex

It's a reasonable argument. Again I think Paolo should make the call.

> > > ---
> > >  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;
> > >