FYI, we noticed the below changes on

https://github.com/jiangliu/linux.git x86_irq_p2v1
commit 16018101c92ad295c51fdc50a8738c46f2f1da81 ("x86/apic: Panic if kernel doesn't support x2apic but BIOS has enabled x2apic")

I think this is the intended the behavior and the kernel message is more clear now :-)

+------------------------------------------------------------------------------------------------------------------------------------+------------+------------+
|                                                                                                                                    | 123a969fab | 16018101c9 |
+------------------------------------------------------------------------------------------------------------------------------------+------------+------------+
| boot_successes                                                                                                                     | 3          | 0          |
| boot_failures                                                                                                                      | 12         | 2          |
| Kernel_panic-not_syncing:IO-APIC+timer_doesn't_work!Boot_with_apic=debug_and_send_a_report.Then_try_booting_with_the'noapic'option | 12         |            |
| backtrace:native_smp_prepare_cpus                                                                                                  | 12         | 1          |
| backtrace:kernel_init_freeable                                                                                                     | 12         | 2          |
| Kernel_panic-not_syncing:BIOS_has_enabled_x2apic_but_kernel_doesn't_support_x2apic,please_disable_x2apic_in_BIOS                   | 0          | 1          |
| Kernel_panic-not_syncing:VFS:Unable_to_mount_root_fs_on_unknown-block(#,#)                                                         | 0          | 1          |
| backtrace:prepare_namespace                                                                                                        | 0          | 1          |
+------------------------------------------------------------------------------------------------------------------------------------+------------+------------+


[    0.233055] smpboot: weird, boot CPU (#255) not listed by the BIOS
[    0.239956] Getting VERSION: ffffffff
[    0.244044] Getting VERSION: ffffffff
[    0.248132] Kernel panic - not syncing: BIOS has enabled x2apic but kernel doesn't support x2apic, please disable x2apic in BIOS.
[    0.248132] 
[    0.262788] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.19.0-rc1-ga1cae60 #1
[    0.270659] Hardware name: Intel Corporation S2600WTT/S2600WTT, BIOS GRNDSDP1.TP2.0025.R02.1403131625 03/13/2014
[    0.282023]  000000000000a180 ffff88085bf8be18 ffffffff81a058d6 00000000000011e2
[    0.290318]  ffffffff81ee7e13 ffff88085bf8be98 ffffffff81a03c39 00000000ffffffff
[    0.298613]  0000000000000008 ffff88085bf8bea8 ffff88085bf8be48 ffff88085bf8be58
[    0.306909] Call Trace:
[    0.309647]  [<ffffffff81a058d6>] dump_stack+0x4c/0x65
[    0.315386]  [<ffffffff81a03c39>] panic+0xc9/0x1f7
[    0.320738]  [<ffffffff82395dac>] enable_IR_x2apic+0x2b/0x2d
[    0.327051]  [<ffffffff82397983>] default_setup_apic_routing+0x12/0x6b
[    0.334339]  [<ffffffff82393d2a>] native_smp_prepare_cpus+0x257/0x360
[    0.341533]  [<ffffffff8238203a>] kernel_init_freeable+0xfc/0x23f
[    0.348337]  [<ffffffff819fc944>] ? rest_init+0x87/0x87
[    0.354169]  [<ffffffff819fc952>] kernel_init+0xe/0xdf
[    0.359907]  [<ffffffff81a0ccbc>] ret_from_fork+0x7c/0xb0
[    0.365934]  [<ffffffff819fc944>] ? rest_init+0x87/0x87
ACPI MEMORY or I/O RESET_REG.
APPEND dmesg: /result/lkp-hsw01/boot/performance-1/debian-x86_64.cgz/x86_64-lkp/a1cae60fa995d086df06d38695cbdf8b5ec87e73/0/.dmesg already exists
early console in decompress_kernel

Thanks,
Huang, Ying