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]).