Re: [Qemu-devel] [BUG/RFC] Two cpus are not brought up normally in SLES11 sp3 VM after reboot

[zhanghailiang <zhang.zhanghailiang@huawei.com>]

On 2015/7/6 16:45, Paolo Bonzini wrote:
>
>
> On 06/07/2015 09:54, zhanghailiang wrote:
>>
>>  From host, we found that QEMU vcpu1 thread and vcpu7 thread were not
>> consuming any cpu (Should be in idle state),
>> All of VCPUs' stacks in host is like bellow:
>>
>> [<ffffffffa07089b5>] kvm_vcpu_block+0x65/0xa0 [kvm]
>> [<ffffffffa071c7c1>] __vcpu_run+0xd1/0x260 [kvm]
>> [<ffffffffa071d508>] kvm_arch_vcpu_ioctl_run+0x68/0x1a0 [kvm]
>> [<ffffffffa0709cee>] kvm_vcpu_ioctl+0x38e/0x580 [kvm]
>> [<ffffffff8116be8b>] do_vfs_ioctl+0x8b/0x3b0
>> [<ffffffff8116c251>] sys_ioctl+0xa1/0xb0
>> [<ffffffff81468092>] system_call_fastpath+0x16/0x1b
>> [<00002ab9fe1f99a7>] 0x2ab9fe1f99a7
>> [<ffffffffffffffff>] 0xffffffffffffffff
>>
>> We looked into the kernel codes that could leading to the above 'Stuck'
>> warning,
>> and found that the only possible is the emulation of 'cpuid' instruct in
>> kvm/qemu has something wrong.
>> But since we can’t reproduce this problem, we are not quite sure.
>> Is there any possible that the cupid emulation in kvm/qemu has some bug ?
>
> Can you explain the relationship to the cpuid emulation?  What do the
> traces say about vcpus 1 and 7?

OK, we searched the VM's kernel codes with the 'Stuck' message, and  it is located in
do_boot_cpu(). It's in BSP context, the call process is:
BSP executes start_kernel() -> smp_init() -> smp_boot_cpus() -> do_boot_cpu() -> wakeup_secondary_via_INIT() to trigger APs.
It will wait 5s for APs to startup, if some AP not startup normally, it will print 'CPU%d Stuck' or 'CPU%d: Not responding'.

If it prints 'Stuck', it means the AP has received the SIPI interrupt and begins to execute the code
'ENTRY(trampoline_data)' (trampoline_64.S) , but be stuck in some places before smp_callin()(smpboot.c).
The follow is the starup process of BSP and AP.
BSP:
start_kernel()
   ->smp_init()
      ->smp_boot_cpus()
        ->do_boot_cpu()
            ->start_ip = trampoline_address(); //set the address that AP will go to execute
            ->wakeup_secondary_cpu_via_init(); // kick the secondary CPU
            ->for (timeout = 0; timeout < 50000; timeout++)
                if (cpumask_test_cpu(cpu, cpu_callin_mask)) break;// check if AP startup or not

APs:
ENTRY(trampoline_data) (trampoline_64.S)
       ->ENTRY(secondary_startup_64) (head_64.S)
          ->start_secondary() (smpboot.c)
             ->cpu_init();
             ->smp_callin();
                 ->cpumask_set_cpu(cpuid, cpu_callin_mask); ->Note: if AP comes here, the BSP will not prints the error message.

 From above call process, we can be sure that, the AP has been stuck between trampoline_data and the cpumask_set_cpu() in
smp_callin(), we look through these codes path carefully, and only found a 'hlt' instruct that could block the process.
It is located in trampoline_data():

ENTRY(trampoline_data)
         ...

	call	verify_cpu		# Verify the cpu supports long mode
	testl   %eax, %eax		# Check for return code
	jnz	no_longmode

         ...

no_longmode:
	hlt
	jmp no_longmode

For the process verify_cpu(),
we can only find the 'cpuid' sensitive instruct that could lead VM exit from No-root mode.
This is why we doubt if cpuid emulation is wrong in KVM/QEMU that leading to the fail in verify_cpu.

 From the message in VM, we know vcpu1 and vcpu7 is something wrong.
[    5.060042] CPU1: Stuck ??
[   10.170815] CPU7: Stuck ??
[   10.171648] Brought up 6 CPUs

Besides, the follow is the cpus message got from host.
80FF72F5-FF6D-E411-A8C8-000000821800:/home/fsp/hrg # virsh qemu-monitor-command instance-0000000
* CPU #0: pc=0x00007f64160c683d thread_id=68570
   CPU #1: pc=0xffffffff810301f1 (halted) thread_id=68573
   CPU #2: pc=0xffffffff810301e2 (halted) thread_id=68575
   CPU #3: pc=0xffffffff810301e2 (halted) thread_id=68576
   CPU #4: pc=0xffffffff810301e2 (halted) thread_id=68577
   CPU #5: pc=0xffffffff810301e2 (halted) thread_id=68578
   CPU #6: pc=0xffffffff810301e2 (halted) thread_id=68583
   CPU #7: pc=0xffffffff810301f1 (halted) thread_id=68584

Oh, i also forgot to mention in the above message that, we have bond each vCPU to different physical CPU in
host.

Thanks,
zhanghailiang