On 2017/8/8 21:57, Paolo Bonzini wrote: > 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. > I understand now, thanks for your patience. :) -- Regards, Longpeng(Mike) > Paolo > >> 4294967295/1000/3600/24 = 49 >> >>> >>> Paolo >>> >> >>> . >>> >> >> > > > . > -- Regards, Longpeng(Mike)