Date: Fri, 3 May 2019 15:42:06 -0300
From: Eduardo Habkost <ehabkost@redhat.com>
Message-ID: <20190503184206.GU28722@habkost.net>
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Subject: Re: [Qemu-devel] [PATCH v2 1/3] q35: set split kernel irqchip as
 default
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Igor Mammedov <imammedo@redhat.com>, "Michael S . Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, Peter Xu <peterx@redhat.com>

On Mon, Apr 29, 2019 at 09:22:12AM -0600, Alex Williamson wrote:
[...]
> > > What's a good 4.0.1 strategy to resolve this?  Re-instate KVM irqchip
> > > as the Q35 default?  I can't see that simply switching to current QEMU
> > > handling is a viable option for performance?  What about 4.1?  We could
> > > certainly improve EOI support in QEMU, there's essentially no support
> > > currently, but it seems like an uphill battle for an iothread based
> > > userspace ioapic to ever compare to KVM handling?  Thanks,  
> > 
> > irqchip=split and irqchip=kernel aren't guest ABI compatible, are
> > they?  That would make it impossible to fix this in pc-q35-4.0
> > for a 4.0.1 update.
> 
> I suppose it would require a pc-q35-4.0.1 machine type :-\  Thanks,

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.

-- 
Eduardo