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

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Message-ID: <MWHPR21MB0863B35957EE6D2869B379ACBFF31@MWHPR21MB0863.namprd21.prod.outlook.com>
Date:   Wed, 2 Dec 2020 19:53:03 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>,
        "x86@...nel.org" <x86@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        vkuznets <vkuznets@...hat.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
Subject: RE: [PATCH] iommu/hyper-v: Fix panic on a host without the 15-bit
 APIC ID support

> From: Thomas Gleixner <tglx@...utronix.de>
> 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