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

[Peter Xu <peterx@redhat.com>]

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>

Hi, Alex,

I have two (probably naive) questions about the patch, possibly due to
lack of context of previous discussions so please let me know if
there's any upstream discussion that I can read.

Firstly, could I ask why we need this 4.0.1 machine type specific for
fixing this problem?  Asked because this seems to be the first time
QEMU introduces the X.Y.Z machine type in master.  Could it be somehow
delayed to the release of QEMU 4.1?  From the planning page I see that
it's releasing on Aug 06th/13th, a bit far away but not really that
much imho.  I'm perfectly fine with this, but I just want to make sure
I have the correct understanding of the motivations.

The second question is about our previous decision to introduce QEMU
4.1 machine type before it's released (which is not related to the
patch at all).  Is it really correct to do so before releasing of 4.1?
So now even with a development QEMU 4.0 branch the user will be able
to create 4.1 machines using "-M pc-q35-4.1", then what if the user
migrated a real 4.1 machine (with the to-be-released QEMU 4.1 binary)
to some 4.1 machine that was run with such an old 4.0 QEMU binary?
The problem is we can add more compatible properties into
pc_q35_4_1_machine_options and future pc_compat_4_1 array before QEMU
4.1 is finally released and then "-M pc-q35-4.1" will actually have
different combination of properties IMHO, which seems to break
compatibility.  Am I wrong somewhere?

Thanks,

-- 
Peter Xu