Messages in this thread | | | Date | Mon, 31 Jan 2022 13:09:26 +0100 | From | Petr Tesařík <> | Subject | Re: [PATCH 1/1] KVM: s390: index kvm->arch.idle_mask by vcpu_idx |
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Hi Halil,
Dne 31. 01. 22 v 12:53 Halil Pasic napsal(a): > On Mon, 31 Jan 2022 11:13:18 +0100 > Petr Tesařík <ptesarik@suse.cz> wrote: > >> Hi Halil, >> >> Dne 27. 08. 21 v 14:54 Halil Pasic napsal(a): >>> While in practice vcpu->vcpu_idx == vcpu->vcp_id is often true, >>> it may not always be, and we must not rely on this. >>> >>> Currently kvm->arch.idle_mask is indexed by vcpu_id, which implies >>> that code like >>> for_each_set_bit(vcpu_id, kvm->arch.idle_mask, online_vcpus) { >>> vcpu = kvm_get_vcpu(kvm, vcpu_id); >>> do_stuff(vcpu); >>> } >>> is not legit. The trouble is, we do actually use kvm->arch.idle_mask >>> like this. To fix this problem we have two options. Either use >>> kvm_get_vcpu_by_id(vcpu_id), which would loop to find the right vcpu_id, >>> or switch to indexing via vcpu_idx. The latter is preferable for obvious >>> reasons. >> >> I'm just backporting this fix to SLES 12 SP5, and I've noticed that >> there is still this code in __floating_irq_kick(): >> >> /* find idle VCPUs first, then round robin */ >> sigcpu = find_first_bit(fi->idle_mask, online_vcpus); >> /* ... round robin loop removed ... >> dst_vcpu = kvm_get_vcpu(kvm, sigcpu); >> >> It seems to me that this does exactly the thing that is not legit, but >> I'm no expert. Did I miss something? >> > > We made that legit by making the N-th bit in idle_mask correspond to the > vcpu whose vcpu_idx == N. The second argument of kvm_get_vcpu() is the > vcpu_idx. IMHO that ain't super-intuitive because it ain't spelled out. > > So before this was a mismatch (with a vcpu_id based bitmap we would have > to use kvm_get_vcpu_by_id()), and with this patch applied this code > becomes legit because both idle_mask and kvm_get_vcpu() operate with > vcpu_idx. > > Does that make sense?
Yes!
> I'm sorry the commit message did not convey this clearly enough...
No, it's not your fault. I didn't pay enough attention to the change, and with vcpu_id and vcpu_idx being so similar I got confused.
In short, there's no bug now, indeed. Thanks for your patience.
Petr T
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