FYI, we noticed the below changes on git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip devel/for-linus-4.1 commit 7a0ea6c5b3ee63edf5d5f1f7bb61ccf964929b08 ("x86/xen: Provide a "Xen PV" APIC driver to support >255 VCPUs") +--------------------------------------------------+------------+------------+ | | dbc36df319 | 7a0ea6c5b3 | +--------------------------------------------------+------------+------------+ | boot_successes | 24 | 0 | | boot_failures | 0 | 30 | | WARNING:at_arch/x86/xen/apic.c:#xen_apic_write() | 0 | 30 | | backtrace:native_smp_prepare_cpus | 0 | 30 | | backtrace:kernel_init_freeable | 0 | 30 | +--------------------------------------------------+------------+------------+ [ 0.047669] ftrace: allocating 31817 entries in 125 pages [ 0.057055] Getting VERSION: 0 [ 0.058004] ------------[ cut here ]------------ [ 0.059017] WARNING: CPU: 0 PID: 1 at arch/x86/xen/apic.c:76 xen_apic_write+0x28/0x30() [ 0.060003] register: 30, value: ff00ff [ 0.060860] Modules linked in: [ 0.061470] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.0.0-rc1-00011-g811bf96 #7 [ 0.062003] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014 [ 0.063003] ffffffff81b7bfa0 ffff880011e63d68 ffffffff818aaa49 0000000000000a08 [ 0.065004] ffff880011e63db8 ffff880011e63da8 ffffffff8107272a 0000000000000000 [ 0.067003] 0000000000000000 000000000000a108 000000000000a110 0000000000000001 [ 0.069003] Call Trace: [ 0.070007] [] dump_stack+0x4c/0x65 [ 0.071006] [] warn_slowpath_common+0x8a/0xc0 [ 0.072005] [] warn_slowpath_fmt+0x46/0x50 [ 0.073005] [] xen_apic_write+0x28/0x30 [ 0.074006] [] verify_local_APIC+0x4e/0x1b7 [ 0.075007] [] native_smp_prepare_cpus+0x221/0x2c7 [ 0.076006] [] kernel_init_freeable+0xfb/0x248 [ 0.077006] [] ? rest_init+0x90/0x90 [ 0.078005] [] kernel_init+0xe/0xf0 [ 0.079005] [] ret_from_fork+0x7c/0xb0 [ 0.080005] [] ? rest_init+0x90/0x90 [ 0.081006] ---[ end trace 4f43b2a59b6f8b66 ]--- [ 0.082003] Getting VERSION: 0 Thanks, Ying Huang