From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Longpeng (Mike)" Subject: Re: [PATCH] KVM: X86: expand ->arch.apic_arb_prio to u64 Date: Wed, 9 Aug 2017 08:59:30 +0800 Message-ID: <598A5E72.2050905@huawei.com> References: <1502192234-14068-1-git-send-email-longpeng2@huawei.com> <5e5d4edc-5861-b3d9-b4a2-890c9bbd53e0@redhat.com> <5989C1B5.5090308@huawei.com> <09780c43-df1f-1f10-bb01-1fc7913910e4@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: , , , , , , To: Paolo Bonzini Return-path: In-Reply-To: <09780c43-df1f-1f10-bb01-1fc7913910e4@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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: >>>> [] dump_stack+0x1e/0x20 >>>> [] ubsan_epilogue+0x12/0x55 >>>> [] handle_overflow+0x1ba/0x215 >>>> [] __ubsan_handle_add_overflow+0x2a/0x31 >>>> [] __apic_accept_irq+0x57a/0x5d0 [kvm] >>>> [] kvm_apic_set_irq+0x9f/0xf0 [kvm] >>>> [] kvm_irq_delivery_to_apic_fast+0x450/0x910 [kvm] >>>> [] kvm_irq_delivery_to_apic+0xfa/0x7a0 [kvm] >>>> [] kvm_set_msi+0xa9/0x100 [kvm] >>>> [] kvm_send_userspace_msi+0x14d/0x1f0 [kvm] >>>> [] 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)