Re: [Qemu-devel] [PATCH for-4.0.1] q35: Revert to kernel irqchip

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 14 May 2019 20:22:32 +0100
Daniel P. Berrangé <berrange@redhat.com> wrote:

> 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.

Yes, I overlooked 4.0.1 on 4.1, I'm working on that now.  Reposting a
v2 of the 4.1 version shortly.

> 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.

Yes, I'm doing both in parallel, I expect the 4.0.1 support to be
contingent on be the master branch change, thus the second link below.
Thanks,

Alex

> > 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-stable@nongnu.org
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> > 
> > Do we want new stable versions for other archs too or only as needed?  
> 
> 
> 
> Regards,
> Daniel