Re: [PATCH] KVM: x86: Fix potential preemption when get the current kvmclock timestamp

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/05/2017 14:00, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
> 
>  BUG: using __this_cpu_read() in preemptible [00000000] code: qemu-system-x86/2809
>  caller is __this_cpu_preempt_check+0x13/0x20
>  CPU: 2 PID: 2809 Comm: qemu-system-x86 Not tainted 4.11.0+ #13
>  Call Trace:
>   dump_stack+0x99/0xce
>   check_preemption_disabled+0xf5/0x100
>   __this_cpu_preempt_check+0x13/0x20
>   get_kvmclock_ns+0x6f/0x110 [kvm]
>   get_time_ref_counter+0x5d/0x80 [kvm]
>   kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
>   ? kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
>   ? kvm_arch_vcpu_ioctl_run+0xac9/0x1ce0 [kvm]
>   kvm_arch_vcpu_ioctl_run+0x5bf/0x1ce0 [kvm]
>   kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
>   ? kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
>   ? __fget+0xf3/0x210
>   do_vfs_ioctl+0xa4/0x700
>   ? __fget+0x114/0x210
>   SyS_ioctl+0x79/0x90
>   entry_SYSCALL_64_fastpath+0x23/0xc2
>  RIP: 0033:0x7f9d164ed357
>  RSP: 002b:00007f9d0f6768f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
>  RAX: ffffffffffffffda RBX: ffffffffa64d53c3 RCX: 00007f9d164ed357
>  RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 000000000000000d
>  RBP: ffffbb260856bf88 R08: 0000556b2a13eeb0 R09: 0000000000000000
>  R10: 00007f9d080000c8 R11: 0000000000000246 R12: 0000000000000000
>  R13: 00007f9d1853d000 R14: 0000000000000000 R15: 000000000000ae80
>   ? __this_cpu_preempt_check+0x13/0x20
> 
> This can be reproduced by run kvm-unit-tests/hyperv_stimer.flat w/ 
> CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT enabled.
> 
> Safe access to per-CPU data requires a couple of constraints, though: the 
> thread working with the data cannot be preempted and it cannot be migrated 
> while it manipulates per-CPU variables. If the thread is preempted, the 
> thread that replaces it could try to work with the same variables; migration 
> to another CPU could also cause confusion. However there is no preemption 
> disable when reads host per-CPU tsc rate to calculate the current kvmclock 
> timestamp.
> 
> This patch fix it by holding pvclock_gtod_sync_lock lock when calculates 
> pvclock's time scale in order to disable preemption for host per-CPU tsc 
> rate read.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b54125b..8008d56 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1772,11 +1772,11 @@ u64 get_kvmclock_ns(struct kvm *kvm)
>  
>  	hv_clock.tsc_timestamp = ka->master_cycle_now;
>  	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
> -	spin_unlock(&ka->pvclock_gtod_sync_lock);
>  
>  	kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
>  			   &hv_clock.tsc_shift,
>  			   &hv_clock.tsc_to_system_mul);
> +	spin_unlock(&ka->pvclock_gtod_sync_lock);
>  	return __pvclock_read_cycles(&hv_clock, rdtsc());
>  }
>  
> 

This would not be enough for PREEMPT_RT.  You need to use
get_cpu/put_cpu (including __pvclock_read_cycles in the non-preemptable
section).

Thanks,

Paolo