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 >> > >> . >> > >
