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From:   Dexuan Cui <decui@microsoft.com>
To:     Thomas Gleixner <tglx@linutronix.de>,
        "dwmw@amazon.co.uk" <dwmw@amazon.co.uk>,
        "x86@kernel.org" <x86@kernel.org>,
        Michael Kelley <mikelley@microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>,
        vkuznets <vkuznets@redhat.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>
Subject: RE: [PATCH] iommu/hyper-v: Fix panic on a host without the 15-bit
 APIC ID support
Thread-Topic: [PATCH] iommu/hyper-v: Fix panic on a host without the 15-bit
 APIC ID support
Thread-Index: AQHWyJFnhI2DigaJq0ylxsLVtcyCgankN98w
Date:   Wed, 2 Dec 2020 19:53:03 +0000
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 <87wny0edko.fsf@nanos.tec.linutronix.de>
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> From: Thomas Gleixner <tglx@linutronix.de>
> Sent: Wednesday, December 2, 2020 1:56 AM
>=20
> 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!
>=20
> This has nothing to do with the 15bit APIC ID support, really.
>=20
> 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.
>=20
> > 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.
>=20
> We don't fix a panic. We fix a bug in the code :)
>=20
> I'll amend the changelog.
>=20
> Thanks,
>=20
>         tglx

Thank you for reworking the commit log, tglx!

Dexuan