linux-kernel - Re: [PATCH] KVM: x86: check for pic and ioapic presence before use

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Date:   Tue, 20 Dec 2016 19:59:55 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, stable@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Steve Rutherford <srutherford@...gle.com>
Subject: Re: [PATCH] KVM: x86: check for pic and ioapic presence before use

2016-11-24 20:42 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2016-11-23 22:58+0100, Paolo Bonzini:
>> On 23/11/2016 21:25, Radim Krčmář wrote:
>>> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
>>> index 25810b144b58..ddd63b8b176e 100644
>>> --- a/arch/x86/kvm/irq_comm.c
>>> +++ b/arch/x86/kvm/irq_comm.c
>>> @@ -41,6 +41,15 @@ static int kvm_set_pic_irq(struct kvm_kernel_irq_routing_entry *e,
>>>                         bool line_status)
>>>  {
>>>      struct kvm_pic *pic = pic_irqchip(kvm);
>>> +
>>> +    /*
>>> +     * XXX: rejecting pic routes when pic isn't in use would be better,
>>> +     * but the default routing table is installed while kvm->arch.vpic is
>>> +     * NULL and KVM_CREATE_IRQCHIP can race with KVM_SET_GSI_ROUTING.
>>> +     */
>>> +    if (!pic)
>>> +            return -1;
>>> +
>>>      return kvm_pic_set_irq(pic, e->irqchip.pin, irq_source_id, level);
>>>  }
>>>
>>
>> Can you explain the race with the default routing table better?  It
>> seems to me that it can only make the routing table go from invalid to
>> valid (there is no KVM_DESTROY_IRQCHIP) so it's benign.
>
> Oops, I wrote the race with wrong IOCTL -- it should be KVM_IRQ_LINE.
>
>  1) set KVM_CAP_SPLIT_IRQCHIP (unlocks KVM_IRQ_LINE)
>  a) call KVM_CREATE_IRQCHIP (creates routes while !kvm->arch.vpic)
>  b) concurrently call KVM_IRQ_LINE for PIO routes (dereferences NULL)

If we should not go through irqfd if irqchip is split?

Regards,
Wanpeng Li