Re: [PATCH] iommu/hyper-v: Fix panic on a host without the 15-bit APIC ID support

[Thomas Gleixner]

On Tue, Dec 01 2020 at 16:45, Dexuan Cui wrote:
> The commit f36a74b9345a itself is good, but it causes a panic in a
> Linux VM that runs on a Hyper-V host that doesn't have the 15-bit
> Extended APIC ID support:
>     kernel BUG at arch/x86/kernel/apic/io_apic.c:2408!

This has nothing to do with the 15bit APIC ID support, really.

The point is that the select() function only matches when I/O APIC ID is
0, which is not guaranteed. That's independent of the 15bit extended
APIC ID feature. But the I/O-APIC ID is irrelevant on hyperv because
there is only one.

> This happens because the Hyper-V ioapic_ir_domain (which is defined in
> drivers/iommu/hyperv-iommu.c) can not be found. Fix the panic by
> properly claiming the only I/O APIC emulated by Hyper-V.

We don't fix a panic. We fix a bug in the code :)

I'll amend the changelog.

Thanks,

        tglx