On Mon, 31 Jan 2022 13:09:26 +0100 Petr Tesařík ptesarik@suse.cz wrote:
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.
No problem at all!
In short, there's no bug now, indeed. Thanks for your patience.
Thank you for being mindful when backporting!
Regards, Halil