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

[Dexuan Cui]

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Wednesday, December 2, 2020 1:56 AM
> 
> 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

Thank you for reworking the commit log, tglx!

Dexuan