Re: [Qemu-devel] [PATCH v2 1/3] q35: set split kernel irqchip as default

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

On Thu, 20 Dec 2018 13:40:35 +0800
Peter Xu <peterx@redhat.com> wrote:

> Starting from QEMU 4.0, let's specify "split" as the default value for
> kernel-irqchip.
> 
> So for QEMU>=4.0 we'll have: allowed=Y,required=N,split=Y
>    for QEMU<=3.1 we'll have: allowed=Y,required=N,split=N
>    (omitting all the "kernel_irqchip_" prefix)
> 
> Note that this will let the default q35 machine type to depend on
> Linux version 4.4 or newer because that's where split irqchip is
> introduced in kernel.  But it's fine since we're boosting supported
> Linux version for QEMU 4.0 to around Linux 4.5.  For more information
> please refer to the discussion on AMD's RDTSCP:
> 
>   https://lore.kernel.org/lkml/20181210181328.GA762@zn.tnic/

It looks like this broke INTx for vfio-pci, see:

https://bugs.launchpad.net/qemu/+bug/1826422

In my testing it looks like KVM advertises supporting the KVM_IRQFD
resample feature, but vfio never gets the unmask notification, so the
device remains with DisINTx set and no further interrupts are
generated.  Do we expect KVM's IRQFD with resampler to work in the
split IRQ mode?  We can certainly hope that "high performance" devices
use MSI or MSI/X, but this would be quite a performance regression with
split mode if our userspace bypass for INTx goes away.  Thanks,

Alex

PS - the least impact workaround for users is to re-enable kernel mode
irqchip with kernel_irqchip=on or <ioapic driver='kvm'/>.  We can also
force QEMU routing of INTx with the x-no-kvm-intx=on option per
vfio-pci device, but this disables the userspace bypass and brings
along higher interrupt latency and overhead.