Re: [PATCH v3] KVM: X86: Fix softlockup when get the current kvmclock

[David Hildenbrand]

On 09.11.2017 03:52, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> 
>  watchdog: BUG: soft lockup - CPU#6 stuck for 22s! [qemu-system-x86:10185]
>  CPU: 6 PID: 10185 Comm: qemu-system-x86 Tainted: G           OE   4.14.0-rc4+ #4
>  RIP: 0010:kvm_get_time_scale+0x4e/0xa0 [kvm]
>  Call Trace:
>   ? get_kvmclock_ns+0xa3/0x140 [kvm]
>   get_time_ref_counter+0x5a/0x80 [kvm]
>   kvm_hv_process_stimers+0x120/0x5f0 [kvm]
>   ? kvm_hv_process_stimers+0x120/0x5f0 [kvm]
>   ? preempt_schedule+0x27/0x30
>   ? ___preempt_schedule+0x16/0x18
>   kvm_arch_vcpu_ioctl_run+0x4b4/0x1690 [kvm]
>   ? kvm_arch_vcpu_load+0x47/0x230 [kvm]
>   kvm_vcpu_ioctl+0x33a/0x620 [kvm]
>   ? kvm_vcpu_ioctl+0x33a/0x620 [kvm]
>   ? kvm_vm_ioctl_check_extension_generic+0x3b/0x40 [kvm]
>   ? kvm_dev_ioctl+0x279/0x6c0 [kvm]
>   do_vfs_ioctl+0xa1/0x5d0
>   ? __fget+0x73/0xa0
>   SyS_ioctl+0x79/0x90
>   entry_SYSCALL_64_fastpath+0x1e/0xa9
> 
> This can be reproduced when running kvm-unit-tests/hyperv_stimer.flat and
> cpu-hotplug stress simultaneously. __this_cpu_read(cpu_tsc_khz) returns 0
> (set in kvmclock_cpu_down_prep()) when the pCPU is unhotplug which results
> in kvm_get_time_scale() gets into an infinite loop.
> 
> This patch fixes it by treating the unhotplug pCPU as not using master clock.
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03869eb..d61dcce3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1795,10 +1795,13 @@ u64 get_kvmclock_ns(struct kvm *kvm)
>  	/* both __this_cpu_read() and rdtsc() should be on the same cpu */
>  	get_cpu();
>  
> -	kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
> -			   &hv_clock.tsc_shift,
> -			   &hv_clock.tsc_to_system_mul);
> -	ret = __pvclock_read_cycles(&hv_clock, rdtsc());
> +	if (__this_cpu_read(cpu_tsc_khz)) {
> +		kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
> +				   &hv_clock.tsc_shift,
> +				   &hv_clock.tsc_to_system_mul);
> +		ret = __pvclock_read_cycles(&hv_clock, rdtsc());
> +	} else

Can we add a comment like

/* unplugging a physical CPU -> avoid using the master clock */

> +		ret = ktime_get_boot_ns() + ka->kvmclock_offset;
>  
>  	put_cpu();
>  
> 

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

-- 

Thanks,

David / dhildenb