linux-kernel - Re: [PATCH] KVM: X86: expand ->arch.apic_arb

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Date:   Tue, 8 Aug 2017 15:57:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Longpeng (Mike)" <longpeng2@...wei.com>
Cc:     rkrcmar@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, weidong.huang@...wei.com,
        arei.gonglei@...wei.com, wangxinxin.wang@...wei.com,
        longpeng.mike@...il.com
Subject: Re: [PATCH] KVM: X86: expand ->arch.apic_arb_prio to u64

On 08/08/2017 15:50, Longpeng (Mike) wrote:
> 
> 
> On 2017/8/8 21:08, Paolo Bonzini wrote:
> 
>> On 08/08/2017 13:37, Longpeng(Mike) wrote:
>>> Currently 'apic_arb_prio' is int32_t, it's too short for long
>>> time running. In our environment, it overflowed and then the
>>> UBSAN was angry:
>>>
>>> signed integer overflow:
>>> 2147483647 + 1 cannot be represented in type 'int'
>>> CPU: 22 PID: 31237 Comm: qemu-kvm Tainted: ...
>>> ...
>>> Call Trace:
>>>  [<ffffffff81f030b6>] dump_stack+0x1e/0x20
>>>  [<ffffffff81f03173>] ubsan_epilogue+0x12/0x55
>>>  [<ffffffff81f04658>] handle_overflow+0x1ba/0x215
>>>  [<ffffffff81f046dd>] __ubsan_handle_add_overflow+0x2a/0x31
>>>  [<ffffffffa126cb1a>] __apic_accept_irq+0x57a/0x5d0 [kvm]
>>>  [<ffffffffa126d14f>] kvm_apic_set_irq+0x9f/0xf0 [kvm]
>>>  [<ffffffffa126db20>] kvm_irq_delivery_to_apic_fast+0x450/0x910 [kvm]
>>>  [<ffffffffa127d8ea>] kvm_irq_delivery_to_apic+0xfa/0x7a0 [kvm]
>>>  [<ffffffffa127e039>] kvm_set_msi+0xa9/0x100 [kvm]
>>>  [<ffffffffa12871ed>] kvm_send_userspace_msi+0x14d/0x1f0 [kvm]
>>>  [<ffffffffa11ed56e>] kvm_vm_ioctl+0x4ee/0xdd0 [kvm]
>>> ...
>>>
>>> We expand it to u64, this is large enough. Suppose the vcpu receives
>>> 1000 irqs per second, then it won't overflow in 584942417 years.
>>> ( 18446744073709551615/1000/3600/24/365 = 584942417 )
>>
>> Since you only look at the difference, changing it to uint32_t should be
>> enough.
> 
> 
> Hi Paolo,
> 
> I'm afraid uint32_t isn't enough. For 1000 irqs per second, it can only holds
> 49 days ( although the overflow won't cause any corruption ).

What matters is only the difference across 2 vCPUs.

And in fact even 32 bits are probably too many, 16 or even 8 should be
enough because overflowing arb_prio is a good thing.  If you have
delivered millions IRQs to VCPU0 (let's say for a day), and then switch
the interrupt to VCPU1, you don't want to the next day to have
interrupts going to VCPU1 only.  A short warm-up time (a few seconds?)
is acceptable, but then you should have interrupts distributed equally
between VCPU0 and VCPU1.  This can only happen if arb_prio overflows.

Paolo

> 4294967295/1000/3600/24 = 49
> 
>>
>> Paolo
>>
> 
>> .
>>
> 
>