On 19/12/18 22:24, Eduardo Habkost wrote: > 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]). Right, RDTSCP is exactly what came to mind.