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

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Date:   Mon, 31 Jan 2022 13:09:26 +0100
From:   Petr Tesařík <ptesarik@...e.cz>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Michael Mueller <mimu@...ux.ibm.com>
Subject: Re: [PATCH 1/1] KVM: s390: index kvm->arch.idle_mask by vcpu_idx

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@...e.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