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

[David Woodhouse]

On Wed, 2022-03-16 at 10:04 +0100, Igor Mammedov wrote:
> Well, I retested with the latest upstream kernel (both guest and host),
> and adding kvm_enable_x2apic() is not sufficient as guest according
> to your patches in kernel caps max APICID at 255 unless kvm-msi-ext-dest-id
> is enabled. And attempt in enabling kvm-msi-ext-dest-id with kernel-irqchip
> fails.

Correctly so. We need the split irqchip to support kvm-msi-ext-dest-id
which is why there's an explicity check for it.

> So number of usable CPUs in guest stays at legacy level, leaving the rest
> of CPUs in limbo.

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.

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