Re: [PATCH 1/4] target/i386: Fix sanity check on max APIC ID / X2APIC enablement

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2022-03-16 at 05:56 -0400, Michael S. Tsirkin wrote:
> On Wed, Mar 16, 2022 at 09:37:07AM +0000, David Woodhouse wrote:
> > Yep, that's the guest operating system's choice. Not a qemu problem.
> > 
> > Even if you have the split IRQ chip, if you boot a guest without kvm-
> > msi-ext-dest-id support, it'll refuse to use higher CPUs.
> > 
> > Or if you boot a guest without X2APIC support, it'll refuse to use
> > higher CPUs. 
> > 
> > That doesn't mean a user should be *forbidden* from launching qemu in
> > that configuration.
> 
> Well the issue with all these configs which kind of work but not
> the way they were specified is that down the road someone
> creates a VM with this config and then expects us to maintain it
> indefinitely.
> 
> So yes, if we are not sure we can support something properly it is
> better to validate and exit than create a VM guests don't know how
> to treat.

Not entirely sure how to reconcile that with what Daniel said in
https://lore.kernel.org/qemu-devel/Yi9BTkZIM3iZsvdK@redhat.com/ which
was:

> We've generally said QEMU should not reject / block startup of valid
> hardware configurations, based on existance of bugs in certain guest
> OS, if the config would be valid for other guest.

That said, I cannot point at a *specific* example of a guest which can
use the higher CPUs even when it can't direct external interrupts at
them. I worked on making Linux capable of it, as I said, but didn't
pursue that in the end.

I *suspect* Windows might be able to do it, based on the way the
hyperv-iommu works (by cheating and returning -EINVAL when external
interrupts are directed at higher CPUs).



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