On Tue, May 14, 2019 at 02:22:03PM -0600, Alex Williamson wrote: > On Tue, 14 May 2019 13:03:31 -0600 > Alex Williamson <alex.william...@redhat.com> 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 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 > > This link is superseded by a v2 of the mainline patch: > > Link: https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg03338.html > > I believe this patch is still the proper stable backport though. Also > to clarify, this patch should be gated on mainline acceptance of the > link above, but clearly there's no clean cherry-pick between mainline > and stable for this, so I'm proposing them in parallel. Thanks,
Agreed. As long as the 4.1 patch can be accepted, this should be the correct patch for 4.0-stable AFAICT: Reviewed-by: Peter Xu <pet...@redhat.com> Thanks, -- Peter Xu