linux-kernel - Re: [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

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Date:	Tue, 28 Jun 2016 10:45:47 -0700
From:	yunhong jiang <yunhong.jiang@...ux.intel.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Yunhong Jiang <yunhong.jiang@...el.com>
Subject: Re: [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

On Tue, 28 Jun 2016 14:54:19 +0800
Wanpeng Li <kernellwp@...il.com> wrote:

> INFO: rcu_sched detected stalls on CPUs/tasks:
>  1-...: (11800 GPs behind) idle=45d/140000000000000/0 softirq=0/0
> fqs=21663 (detected by 0, t=65016 jiffies, g=11500, c=11499, q=719)
> Task dump for CPU 1:
> qemu-system-x86 R  running task        0  3529   3525 0x00080808
>  ffff8802021791a0 ffff880212895040 0000000000000001 00007f1c2c00db40
>  ffff8801dd20fcd3 ffffc90002b98000 ffff8801dd20fc88 ffff8801dd20fcf8
>  0000000000000286 ffff8801dd2ac538 ffff8801dd20fcc0 ffffffffc06949c9
> Call Trace:
> ? kvm_write_guest_cached+0xb9/0x160 [kvm]
> ? __delay+0xf/0x20
> ? wait_lapic_expire+0x14a/0x200 [kvm]
> ? kvm_arch_vcpu_ioctl_run+0xcbe/0x1b00 [kvm]
> ? kvm_arch_vcpu_ioctl_run+0xe34/0x1b00 [kvm]
> ? kvm_vcpu_ioctl+0x2d3/0x7c0 [kvm]
> ? __fget+0x5/0x210
> ? do_vfs_ioctl+0x96/0x6a0
> ? __fget_light+0x2a/0x90
> ? SyS_ioctl+0x79/0x90
> ? do_syscall_64+0x7c/0x1e0
> ? entry_SYSCALL64_slow_path+0x25/0x25
> 
> This can be reproduced readily by running a full dynticks guest(since
> hrtimer in guest is heavily used) w/ lapic_timer_advance disabled.
> 
> If fail to program hardware preemption timer, we will fallback to
> hrtimer based method, however, a previous programmed preemption timer
> miss to cancel in this scenario which results in one hardware
> preemption timer and one hrtimer emulated tsc deadline timer run
> simultaneously. So sometimes the target guest deadline tsc is earlier
> than guest tsc, which leads to the computation in vmx_set_hv_timer
> can underflow and cause delta_tsc to be set a huge value, then host
> soft lockup as above. 
> 
> This patch fix it by cancelling the previous programmed preemption
> timer if there is once we failed to program the new preemption timer
> and fallback to hrtimer based method.

Hi, WanPeng, thanks for the patch.

> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Yunhong Jiang <yunhong.jiang@...el.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/lapic.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index fdc05ae..b15e32a 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1454,11 +1454,18 @@ static void start_apic_timer(struct kvm_lapic
> *apic) /* lapic timer in tsc deadline mode */
>  		u64 tscdeadline = apic->lapic_timer.tscdeadline;
>  
> -		if (kvm_x86_ops->set_hv_timer &&
> -		    !kvm_x86_ops->set_hv_timer(apic->vcpu,
> tscdeadline)) {
> -			apic->lapic_timer.hv_timer_in_use = true;
> -			trace_kvm_hv_timer_state(apic->vcpu->vcpu_id,
> +		if (kvm_x86_ops->set_hv_timer) {
> +			if (kvm_x86_ops->set_hv_timer(apic->vcpu,

Would it be better that if set_hv_timer fails, we clear the vmx timer (i.e. the
VMCS field) before return the failure? I'm not sure if it make sense to clear
the previous setup if a new setup fails, although it seems OK for me, since we
have to cancel the hv_timer anyway.

Your idea?

Thanks
--jyh

> tscdeadline)) {
> +				if
> (apic->lapic_timer.hv_timer_in_use) {
> +
> kvm_x86_ops->cancel_hv_timer(apic->vcpu);
> +
> apic->lapic_timer.hv_timer_in_use = false;
> +				}
> +				start_sw_tscdeadline(apic);
> +			} else {
> +				apic->lapic_timer.hv_timer_in_use =
> true;
> +
> trace_kvm_hv_timer_state(apic->vcpu->vcpu_id,
> apic->lapic_timer.hv_timer_in_use);
> +			}
>  		} else
>  			start_sw_tscdeadline(apic);
>  	}