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

[Eduardo Habkost]

On Mon, May 06, 2019 at 11:13:28AM -0500, Paolo Bonzini wrote:
> 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.

I don't mean we necessarily should enable IR for
kernel-irqchip=on too.  I'd just prefer the default setting to
not depend on the kernel-irqchip option.

x86-iommu could either have intremap=on as the default (and
refuse to run with kernel-irqchip=on without explicit
intremap=off), or simply default to intremap=off.

But as Paolo indicated elsewhere, this is not the only guest ABI
difference between "on" and "split".  Probably it's not worth the
hassle to try to to untangle this.

-- 
Eduardo