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Subject: Re: [PATCH 1/1] KVM: s390: index kvm->arch.idle_mask by vcpu_idx
To:     Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Halil Pasic <pasic@linux.ibm.com>
Cc:     Janosch Frank <frankja@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Michael Mueller <mimu@linux.ibm.com>
References: <20210827125429.1912577-1-pasic@linux.ibm.com>
 <20210827160616.532d6699@p-imbrenda>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <e9d2f79c-b784-bc6b-88dc-2d0f7cc08dbe@de.ibm.com>
Date:   Fri, 27 Aug 2021 18:36:48 +0200
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On 27.08.21 16:06, Claudio Imbrenda wrote:
> On Fri, 27 Aug 2021 14:54:29 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
> 
>> While in practice vcpu->vcpu_idx ==  vcpu->vcp_id is often true,
>> it may not always be, and we must not rely on this.
> 
> why?
> 
> maybe add a simple explanation of why vcpu_idx and vcpu_id can be
> different, namely:
> KVM decides the vcpu_idx, userspace decides the vcpu_id, thus the two
> might not match
> 
>>
>> 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);
> 
> you can also add a sentence to clarify that kvm_get_vcpu expects an
> vcpu_idx, not an vcpu_id.
> 
>> 		do_stuff(vcpu);

I will modify the patch description accordingly before sending to Paolo.
Thanks for noticing.


>> }
>> 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.
>>
>> Let us make switch from indexing kvm->arch.idle_mask by vcpu_id to
>> indexing it by vcpu_idx.  To keep gisa_int.kicked_mask indexed by the
>> same index as idle_mask lets make the same change for it as well.
>>
>> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
>> Fixes: 1ee0bc559dc3 ("KVM: s390: get rid of local_int array")
> 
> otherwise looks good to me.
> 
> Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> 
>> Cc: <stable@vger.kernel.org> # 3.15+
>> ---
>>   arch/s390/include/asm/kvm_host.h |  1 +
>>   arch/s390/kvm/interrupt.c        | 12 ++++++------
>>   arch/s390/kvm/kvm-s390.c         |  2 +-
>>   arch/s390/kvm/kvm-s390.h         |  2 +-
>>   4 files changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
>> index 161a9e12bfb8..630eab0fa176 100644
>> --- a/arch/s390/include/asm/kvm_host.h
>> +++ b/arch/s390/include/asm/kvm_host.h
>> @@ -957,6 +957,7 @@ struct kvm_arch{
>>   	atomic64_t cmma_dirty_pages;
>>   	/* subset of available cpu features enabled by user space */
>>   	DECLARE_BITMAP(cpu_feat, KVM_S390_VM_CPU_FEAT_NR_BITS);
>> +	/* indexed by vcpu_idx */
>>   	DECLARE_BITMAP(idle_mask, KVM_MAX_VCPUS);
>>   	struct kvm_s390_gisa_interrupt gisa_int;
>>   	struct kvm_s390_pv pv;
>> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
>> index d548d60caed2..16256e17a544 100644
>> --- a/arch/s390/kvm/interrupt.c
>> +++ b/arch/s390/kvm/interrupt.c
>> @@ -419,13 +419,13 @@ static unsigned long deliverable_irqs(struct kvm_vcpu *vcpu)
>>   static void __set_cpu_idle(struct kvm_vcpu *vcpu)
>>   {
>>   	kvm_s390_set_cpuflags(vcpu, CPUSTAT_WAIT);
>> -	set_bit(vcpu->vcpu_id, vcpu->kvm->arch.idle_mask);
>> +	set_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.idle_mask);
>>   }
>>   
>>   static void __unset_cpu_idle(struct kvm_vcpu *vcpu)
>>   {
>>   	kvm_s390_clear_cpuflags(vcpu, CPUSTAT_WAIT);
>> -	clear_bit(vcpu->vcpu_id, vcpu->kvm->arch.idle_mask);
>> +	clear_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.idle_mask);
>>   }
>>   
>>   static void __reset_intercept_indicators(struct kvm_vcpu *vcpu)
>> @@ -3050,18 +3050,18 @@ int kvm_s390_get_irq_state(struct kvm_vcpu *vcpu, __u8 __user *buf, int len)
>>   
>>   static void __airqs_kick_single_vcpu(struct kvm *kvm, u8 deliverable_mask)
>>   {
>> -	int vcpu_id, online_vcpus = atomic_read(&kvm->online_vcpus);
>> +	int vcpu_idx, online_vcpus = atomic_read(&kvm->online_vcpus);
>>   	struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;
>>   	struct kvm_vcpu *vcpu;
>>   
>> -	for_each_set_bit(vcpu_id, kvm->arch.idle_mask, online_vcpus) {
>> -		vcpu = kvm_get_vcpu(kvm, vcpu_id);
>> +	for_each_set_bit(vcpu_idx, kvm->arch.idle_mask, online_vcpus) {
>> +		vcpu = kvm_get_vcpu(kvm, vcpu_idx);
>>   		if (psw_ioint_disabled(vcpu))
>>   			continue;
>>   		deliverable_mask &= (u8)(vcpu->arch.sie_block->gcr[6] >> 24);
>>   		if (deliverable_mask) {
>>   			/* lately kicked but not yet running */
>> -			if (test_and_set_bit(vcpu_id, gi->kicked_mask))
>> +			if (test_and_set_bit(vcpu_idx, gi->kicked_mask))
>>   				return;
>>   			kvm_s390_vcpu_wakeup(vcpu);
>>   			return;
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index 4527ac7b5961..8580543c5bc3 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -4044,7 +4044,7 @@ static int vcpu_pre_run(struct kvm_vcpu *vcpu)
>>   		kvm_s390_patch_guest_per_regs(vcpu);
>>   	}
>>   
>> -	clear_bit(vcpu->vcpu_id, vcpu->kvm->arch.gisa_int.kicked_mask);
>> +	clear_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.gisa_int.kicked_mask);
>>   
>>   	vcpu->arch.sie_block->icptcode = 0;
>>   	cpuflags = atomic_read(&vcpu->arch.sie_block->cpuflags);
>> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
>> index 9fad25109b0d..ecd741ee3276 100644
>> --- a/arch/s390/kvm/kvm-s390.h
>> +++ b/arch/s390/kvm/kvm-s390.h
>> @@ -79,7 +79,7 @@ static inline int is_vcpu_stopped(struct kvm_vcpu *vcpu)
>>   
>>   static inline int is_vcpu_idle(struct kvm_vcpu *vcpu)
>>   {
>> -	return test_bit(vcpu->vcpu_id, vcpu->kvm->arch.idle_mask);
>> +	return test_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.idle_mask);
>>   }
>>   
>>   static inline int kvm_is_ucontrol(struct kvm *kvm)
>>
>> base-commit: 77dd11439b86e3f7990e4c0c9e0b67dca82750ba
>