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JgZu/9FOJszYfLcKXv8/WSxfhiHN1nt+5oo0wuZ3ZcFtdkwQnkwZiade9gl9mB4hUtlZW807C/M teNj+O4CAImL18ggFNzL9LDCgkBvtNpguvKmdQX8= X-Proofpoint-ORIG-GUID: ERLOP2jP2Yb6UqyWesZ5c1qYlb2HyXDS X-Proofpoint-Spam-Info: AW1haW4tMjYwODE3MDE1NSBTYWx0ZWRfX3NfvwqckIChf Rd9QQKyZAfjXUQxinzKJVuoi4kT9myfStK+jdJKahuAxWqEgjWd8PKBEzQEEIk7hhluvXuF/ehI +Q2ln60ZYutrFFlD6RA7wDP+tLZwH9ED0c1vHf4rp1P95YUsqQOJ On Sat, Aug 15, 2026 10:55:25PM -0700, sashiko-bot@kernel.org wrote: > Thank you for your contribution! Sashiko AI review found 2 potential issu= e(s) to > consider: Pre-existing issues: - [High] The patch addresses the steal time > corruption on vCPU PID change for x86 KVM PV but completely misses the > equivalent KVM >=20 >=20 > Thank you for your contribution! Sashiko AI review found 2 potential issu= e(s) to consider: >=20 > Pre-existing issues: > - [High] The patch addresses the steal time corruption on vCPU PID change= for x86 KVM PV but completely misses the equivalent KVM Xen steal time (`v= cpu->arch.xen.last_steal`), leaving it exposed to the identical integer und= erflow. > - [High] The identical vCPU thread migration bug exists in ARM64, RISC-V,= and LoongArch steal time accounting, but the patch only implements the fix= for x86. > -- >=20 > --- Patch [1]: [PATCH 1/4] KVM: x86: Reset last_steal on vCPU pid change = --- > commit d7367ba78784ba11d9b540abce98359a8e902828 > Author: Dongli Zhang >=20 > KVM: x86: Reset last_steal on vCPU pid change >=20 > KVM does not reset last_steal on vCPU re-creation/hotplug, causing a bogus > large steal time due to run_delay starting from zero. This patch fixes it= by > resetting vcpu->arch.st.last_steal in kvm_arch_vcpu_run_pid_change on x86. >=20 >> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig >> index 801bf9e520db3..b7cb2ceda6d9a 100644 >> --- a/arch/x86/kvm/Kconfig >> +++ b/arch/x86/kvm/Kconfig >> @@ -40,6 +40,7 @@ config KVM_X86 >> select HAVE_KVM_MSI >> select HAVE_KVM_CPU_RELAX_INTERCEPT >> select HAVE_KVM_NO_POLL >> + select HAVE_KVM_VCPU_RUN_PID_CHANGE >=20 > [Severity: High] > This is a pre-existing issue, but does this same integer underflow bug ex= ist > on other architectures? >=20 > Looking at ARM64, RISC-V, and LoongArch, they all track steal time using > current->sched_info.run_delay and subtract the old last_steal value, which > appears identically vulnerable to underflow on vCPU thread recreation. >=20 > For example, on ARM64: >=20 > arch/arm64/kvm/pvtime.c:kvm_update_stolen_time() { > ... > steal +=3D vcpu->arch.steal.last_steal - last_steal; > ... > } >=20 > (Note that arm64's kvm_arch_vcpu_run_pid_change() returns early if > vcpu_has_run_once(vcpu) is true, skipping the opportunity to update last_= steal.) >=20 > Are there plans to address this on the other architectures? Patch 2 addresses the arm64 case. risc-v and loongarch may need similar handling, but I didn't include them in this series because I only have x86 and arm64 KVM test environments availab= le to validate the fix. I don't currently have a risc-v or loongarch setup for KVM testing. >=20 >> select VIRT_XFER_TO_GUEST_WORK >> select KVM_GENERIC_DIRTYLOG_READ_PROTECT >> select KVM_VFIO >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index 47cb9eba113b1..33be45eec32bc 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c > [ ... ] >> @@ -3717,6 +3717,13 @@ void kvm_service_local_tlb_flush_requests(struct = kvm_vcpu *vcpu) >> } >> EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_service_local_tlb_flush_requests); >> =20 >> +int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu) >> +{ >> + vcpu->arch.st.last_steal =3D current->sched_info.run_delay; >> + >=20 > [Severity: High] > This isn't a bug introduced by this patch, but should this function also > reset vcpu->arch.xen.last_steal? >=20 > Looking at kvm_xen_update_runstate() in arch/x86/kvm/xen.c, the stolen ti= me > for Xen runstate is calculated similarly: >=20 > arch/x86/kvm/xen.c:kvm_xen_update_runstate() { > ... > u64 steal_ns =3D run_delay - vx->last_steal; > delta_ns -=3D steal_ns; > vx->runstate_times[RUNSTATE_runnable] +=3D steal_ns; > ... > vx->last_steal =3D run_delay; > } >=20 > If the thread changes and run_delay resets to 0, wouldn't vx->last_steal > also cause an identical integer underflow? >=20 As explained in the cover letter, "Although David also suggested doing the = same for Xen-on-KVM vCPUs, this series does not reset last_steal for Xen vCPUs. = That change itself would not be difficult, but Xen uses a different mechanism to account downtime, including runnable time and offline time when a vCPU is n= ot running. It may therefore need no additional ioctl, or a smaller ioctl extension, to account run_delay left over from the previous PID. For now, t= his series changes only regular x86 steal time and arm64 PV time." Dongli Zhang