linux-kernel - Re: [RESEND PATCH 0/3] x86/apic/kexec: Enable legacy irq mode before jump to kexec/kdump kernel

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Date:   Thu, 11 Jan 2018 13:05:56 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, fenghua.yu@...el.com,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        x86@...nel.org, rostedt@...dmis.org, jgross@...e.com,
        peterz@...radead.org, uobergfe@...hat.com, joro@...tes.org
Subject: Re: [RESEND PATCH 0/3] x86/apic/kexec: Enable legacy irq mode before jump to kexec/kdump kernel

Baoquan He <bhe@...hat.com> writes:

> Hi all,
>
> PING!
>
> (Add Fenghua and Eric to this thread)
>
> On 01/05/18 at 11:42am, Baoquan He wrote:
>> On kvm guest, the latest kernel will always print warning during kdump kernel boots
>> as below. The reaons is the legacy irq mode is disabled before jump to kexec/kdump
>> kernel. So in setup_local_APIC(), the do { xxx } while (queued && max_loops > 0)
>> can't handle if pending irq exists in APIC IRR since LAPIC is disabled. It will
>> terminate the do while loop finally when max_loops overflows by subtraction. Then
>> WARN_ON(max_loops <= 0) is triggered.

Overall this looks like the code is setup_local_APIC is working largely
as designed.  It does run into a snag so it warns.

Which leaves the question:  Does QEMU have buggy APIC emulation in this
case or is that loop simply incapble of dealing with queued interrupts
in APIC_IRR.

>> 
>> [    0.001000] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/apic/apic.c:1467 setup_local_APIC+0x228/0x330
>> [    0.001000] Modules linked in:
>> [    0.001000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.15.0-rc5+ #3
>> [    0.001000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1.fc26 04/01/2014
>> [    0.001000] RIP: 0010:setup_local_APIC+0x228/0x330
>> [    0.001000] RSP: 0000:ffffffffb6e03eb8 EFLAGS: 00010286
>> [    0.001000] RAX: 0000009edb4c4d84 RBX: 0000000000000000 RCX: 00000000b099d800
>> [    0.001000] RDX: 0000009e00000000 RSI: 0000000000000000 RDI: 0000000000000810
>> [    0.001000] RBP: 0000000000000000 R08: ffffffffffffffff R09: 0000000000000001
>> [    0.001000] R10: ffff98ce6a801c00 R11: 0761076d072f0776 R12: 0000000000000001
>> [    0.001000] R13: 00000000000000f0 R14: 0000000000004000 R15: ffffffffffffc6ff
>> [    0.001000] FS:  0000000000000000(0000) GS:ffff98ce6bc00000(0000) knlGS:0000000000000000
>> [    0.001000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [    0.001000] CR2: 00000000ffffffff CR3: 0000000022209000 CR4: 00000000000406b0
>> [    0.001000] Call Trace:
>> [    0.001000]  apic_bsp_setup+0x56/0x74
>> [    0.001000]  x86_late_time_init+0x11/0x16
>> [    0.001000]  start_kernel+0x3c9/0x486
>> [    0.001000]  secondary_startup_64+0xa5/0xb0
>> [    0.001000] Code: 00 85 c9 74 2d 0f 31 c1 e1 0a 48 c1 e2 20 41 89 cf 4c 03 7c 24 08 48 09 d0 49 29 c7 4c 89 3c 24 48 83 3c 24 00 0f 8f 8f fe ff ff <0f> ff e9 10 ff ff ff 48 83 2c 24 01 eb e7 48 83 c4 18 5b 5d 41 
>> [    0.001000] ---[ end trace b88e71b9a6ebebdd ]---
>> [    0.001000] masked ExtINT on CPU#0
>> 
>> With patch 2/3 applied, the above warning disappeared. And with patch 2/3
>> applied, the issue mentioned in patch 1/3 can also be fixed because the LAPIC
>> has been set as ExtINT before jump to kdump kernel, while we had better set it
>> explicitly. Seems no reason not to enable legacy irq mode before jump to
>> kexec/kdump kernel, and can make it be consistent with normal kernel.
>> 
>> Patch 3/3 is doing clean up, I am fine if people think it's unnecessary.
>> 

I don't see these patches so I am simply going on their description.

>> Baoquan He (3):
>>   x86/apic: Set up through LAPIC on boot CPU's LINT0 if ioapic is
>>     disabled

*scratches my head*  Are you booging the kexec on panic kernel with
apics disabled?  When the previous kernel had apics enabled?
That makes my head really hurt if you are.  Don't do that.

Eric