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

[Paolo Bonzini]

On 05/05/19 04:06, Peter Xu wrote:
>> I wonder if it's possible to untangle this and make the irqchip
>> option stop affecting guest ABI on 4.1+ machine-types?  This way
>> QEMU could choose smarter defaults in the future without the
>> compatibility code hassle.
> Hi, Eduardo,
> 
> Do you mean to enable IOMMU IR for kernel-irqchip=on?  If so, I would
> say it's not trivial...  The major issue is that we probably need to
> explicitly kick QEMU for every kernel IOAPIC setups since QEMU is the
> only one who knows everything about interrupt remapping, while KVM
> don't even have such a mechanism so far.

Right, it's not easy and it would be anyway possible only on kernels.
There would have to be a mechanism to setup IOAPIC->MSI routes, similar
to irqchip=split, and as Peter mentions an MMIO exit on writes to the
routing table.

Paolo