On Tue, May 14, 2019 at 01:03:31PM -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 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.

If we introduce a pc-q35-4.0.1 machine type in -stable, then VMs
created in stable won't be migratable to future 4.1 unless we also
create this same machine type in master.

If we really want to create a new 4.0.1 machine type, then this
patch needs to go to git master before stable IMHO to guarantee
no regression to master.

> Link: https://bugs.launchpad.net/qemu/+bug/1826422
> Link: https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg03305.html
> Fixes: b2fc91db8447 ("q35: set split kernel irqchip as default")
> Cc: qemu-sta...@nongnu.org
> Signed-off-by: Alex Williamson <alex.william...@redhat.com>
> ---
> Do we want new stable versions for other archs too or only as needed?

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