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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jun 03, 2019 at 10:06:06AM +0200, Paolo Bonzini wrote:
> On 29/05/19 05:30, Michael S. Tsirkin 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.
> 
> Yeah, me too but I don't see an alternative.  I'll merge it today.

FYI in Fedora we've had another unrelated regression bug that was identified
as caused by the split irqchip change. With a Windows 7 guest, the clock
is way too fast, for every 1 second wallclock time, 15 seconds passes
in the guest. See the bug for more info:

  https://bugzilla.redhat.com/show_bug.cgi?id=1704375

Regards,
Daniel
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