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Re: [libvirt] [PATCH 2/4] q35: set split kernel irqchip as default



On Wed, Dec 19, 2018 at 09:12:55PM +0100, Paolo Bonzini wrote:
> On 19/12/18 09:50, Peter Xu 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 "split" is optional - we'll first try to enable split
> > kernel irqchip, and we'll fall back to complete kernel irqchip if we
> > found that the kernel capability is missing.
> 
> Please just fail completely and require a new kernel for the 4.0 machine
> type.  There are subtle differences between kernel and QEMU irqchip, I
> don't think we want to open that can of worms.

This would make existing VMs that are runnable with pc-q35-3.1.0
not runnable by only updating the machine-type.

The good news is that we can make this a non-issue by clearly
documenting that QEMU needs a more recent kernel (just like we'll
do for RDTSCP[1]).

[1] https://lore.kernel.org/lkml/20181210181328 GA762 zn tnic/

-- 
Eduardo


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