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HVM guest only bring up a single vCPU



Hi Andrew,

While doing more testing today, I noticed that only one vCPU would be brought up with HVM guest with Xen 4.16 on my setup (QEMU):

[ 1.122180] ================================================================================ [ 1.122180] UBSAN: shift-out-of-bounds in oss/linux/arch/x86/kernel/apic/apic.c:2362:13
[    1.122180] shift exponent -1 is negative
[    1.122180] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.14.0-rc7+ #304
[    1.122180] Hardware name: Xen HVM domU, BIOS 4.16-unstable 06/07/2021
[    1.122180] Call Trace:
[    1.122180]  dump_stack_lvl+0x56/0x6c
[    1.122180]  ubsan_epilogue+0x5/0x50
[    1.122180]  __ubsan_handle_shift_out_of_bounds+0xfa/0x140
[    1.122180]  ? cgroup_kill_write+0x4d/0x150
[    1.122180]  ? cpu_up+0x6e/0x100
[    1.122180]  ? _raw_spin_unlock_irqrestore+0x30/0x50
[    1.122180]  ? rcu_read_lock_held_common+0xe/0x40
[    1.122180]  ? irq_shutdown_and_deactivate+0x11/0x30
[    1.122180]  ? lock_release+0xc7/0x2a0
[    1.122180]  ? apic_id_is_primary_thread+0x56/0x60
[    1.122180]  apic_id_is_primary_thread+0x56/0x60
[    1.122180]  cpu_up+0xbd/0x100
[    1.122180]  bringup_nonboot_cpus+0x4f/0x60
[    1.122180]  smp_init+0x26/0x74
[    1.122180]  kernel_init_freeable+0x183/0x32d
[    1.122180]  ? _raw_spin_unlock_irq+0x24/0x40
[    1.122180]  ? rest_init+0x330/0x330
[    1.122180]  kernel_init+0x17/0x140
[    1.122180]  ? rest_init+0x330/0x330
[    1.122180]  ret_from_fork+0x22/0x30
[ 1.122244] ================================================================================
[    1.123176] installing Xen timer for CPU 1
[    1.123369] x86: Booting SMP configuration:
[    1.123409] .... node  #0, CPUs:      #1
[    1.154400] Callback from call_rcu_tasks_trace() invoked.
[    1.154491] smp: Brought up 1 node, 1 CPU
[    1.154526] smpboot: Max logical packages: 2
[    1.154570] smpboot: Total of 1 processors activated (5999.99 BogoMIPS)

I have tried a PV guest (same setup) and the kernel could bring up all the vCPUs.

Digging down, Linux will set smp_num_siblings to 0 (via detect_ht_early()) and as a result will skip all the CPUs. The value is retrieve from a CPUID leaf. So it sounds like we don't set the leaft correctly.

FWIW, I have also tried on Xen 4.11 and could spot the same issue. Does this ring any bell to you?

Cheers,

--
Julien Grall



 


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