Re: [PATCH 1/1] KVM: s390: index kvm->arch.idle_mask by vcpu_idx

[Halil Pasic <pasic@linux.ibm.com>]

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