Paolo? Please see MST's request below. Thanks,
Alex On Wed, 29 May 2019 12:01:14 -0400 "Michael S. Tsirkin" <m...@redhat.com> wrote: > 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" <m...@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.william...@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; > > > >