Re: [Qemu-devel] [PATCH 2/4] q35: set split kernel irqchip as default

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo