linux-kernel - Re: [PATCH] KVM: LAPIC: Also cancel preemption timer during SET

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Date:   Tue, 25 Jan 2022 12:04:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Also cancel preemption timer during SET_LAPIC

On 1/25/22 10:17, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> The below warning is splatting during guest reboot.
> 
>    ------------[ cut here ]------------
>    WARNING: CPU: 0 PID: 1931 at arch/x86/kvm/x86.c:10322 kvm_arch_vcpu_ioctl_run+0x874/0x880 [kvm]
>    CPU: 0 PID: 1931 Comm: qemu-system-x86 Tainted: G          I       5.17.0-rc1+ #5
>    RIP: 0010:kvm_arch_vcpu_ioctl_run+0x874/0x880 [kvm]
>    Call Trace:
>     <TASK>
>     kvm_vcpu_ioctl+0x279/0x710 [kvm]
>     __x64_sys_ioctl+0x83/0xb0
>     do_syscall_64+0x3b/0xc0
>     entry_SYSCALL_64_after_hwframe+0x44/0xae
>    RIP: 0033:0x7fd39797350b
> 
> This can be triggered by not exposing tsc-deadline mode and doing a reboot in
> the guest. The lapic_shutdown() function which is called in sys_reboot path
> will not disarm the flying timer, it just masks LVTT. lapic_shutdown() clears
> APIC state w/ LVT_MASKED and timer-mode bit is 0, this can trigger timer-mode
> switch between tsc-deadline and oneshot/periodic, which can result in preemption
> timer be cancelled in apic_update_lvtt(). However, We can't depend on this when
> not exposing tsc-deadline mode and oneshot/periodic modes emulated by preemption
> timer. Qemu will synchronise states around reset, let's cancel preemption timer
> under KVM_SET_LAPIC.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>   arch/x86/kvm/lapic.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index baca9fa37a91..4662469240bc 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2629,7 +2629,7 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>   	kvm_apic_set_version(vcpu);
>   
>   	apic_update_ppr(apic);
> -	hrtimer_cancel(&apic->lapic_timer.timer);
> +	cancel_apic_timer(apic);
>   	apic->lapic_timer.expired_tscdeadline = 0;
>   	apic_update_lvtt(apic);
>   	apic_manage_nmi_watchdog(apic, kvm_lapic_get_reg(apic, APIC_LVT0));

Queued, with Cc to stable.

Paolo