linux-kernel - Re: [PATCH v5 10/15] KVM: s390: add functions to (un)register GISC with GISA

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Date:   Tue, 8 Jan 2019 14:07:06 +0100
From:   Michael Mueller <mimu@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>,
        KVM Mailing List <kvm@...r.kernel.org>,
        Linux-S390 Mailing List <linux-s390@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH v5 10/15] KVM: s390: add functions to (un)register GISC
 with GISA



On 08.01.19 11:34, Cornelia Huck wrote:
> On Mon, 7 Jan 2019 18:38:02 +0100
> Michael Mueller <mimu@...ux.ibm.com> wrote:
> 
>> On 04.01.19 14:19, Cornelia Huck wrote:
>>> On Wed, 2 Jan 2019 18:29:00 +0100
>>> Pierre Morel <pmorel@...ux.ibm.com> wrote:
>>>    
>>>> On 19/12/2018 20:17, Michael Mueller wrote:
>>>>> Add the IAM (Interruption Alert Mask) to the architecture specific
>>>>> kvm struct. This mask in the GISA is used to define for which ISC
>>>>> a GIB alert can be issued.
>>>>>
>>>>> The functions kvm_s390_gisc_register() and kvm_s390_gisc_unregister()
>>>>> are used to (un)register a GISC (guest ISC) with a virtual machine and
>>>>> its GISA.
>>>>>
>>>>> Upon successful completion, kvm_s390_gisc_register() returns the
>>>>> ISC to be used for GIB alert interruptions. A negative return code
>>>>> indicates an error during registration.
>>>>>
>>>>> Theses functions will be used by other adapter types like AP and PCI to
>>>>> request pass-through interruption support.
>>>>>
>>>>> Signed-off-by: Michael Mueller <mimu@...ux.ibm.com>
>>>>> ---
>>>>>     arch/s390/include/asm/kvm_host.h |  9 ++++++
>>>>>     arch/s390/kvm/interrupt.c        | 66 ++++++++++++++++++++++++++++++++++++++++
>>>>>     2 files changed, 75 insertions(+)
>>>>>   
>>>    
>>>>> +int kvm_s390_gisc_register(struct kvm *kvm, u32 gisc)
>>>>> +{
>>>>> +	if (!kvm->arch.gib_in_use)
>>>>> +		return -ENODEV;
>>>>> +	if (gisc > MAX_ISC)
>>>>> +		return -ERANGE;
>>>>> +
>>>>> +	spin_lock(&kvm->arch.iam_ref_lock);
>>>>> +	if (kvm->arch.iam_ref_count[gisc] == 0)
>>>>> +		kvm->arch.iam |= 0x80 >> gisc;
>>>>> +	kvm->arch.iam_ref_count[gisc]++;
>>>>> +	if (kvm->arch.iam_ref_count[gisc] == 1)
>>>>> +		set_iam(kvm->arch.gisa, kvm->arch.iam);
>>>>
>>>> testing the set_iam return value?
>>>> Even it should be fine if the caller works correctly, this is done
>>>> before GISA is ever used.
>>
>> There is a rc but a check here is not required.
>>
>> There are three cases:
>>
>> a) This is the first ISC that gets registered, then the GISA is
>>      not in use and IAM is set in the GISA.
>>
>> b) A second ISC gets registered and the GISA is *not* in the
>>      alert list. Then the IAM is set here as well.
>>
>> c) A second ISC gets registered and the GISA is in the
>>      alert list. Then the IAM is intentionally not set here
>>      by set_iam(). It will be restored by get_ipm() with
>>      the new IAM value by the gib alert processing code.
>>
>>
>>>
>>> My feeling is that checking the return code is a good idea, even if it
>>> Should Never Fail(tm).
>>>    
>>>>   
>>>>> +	spin_unlock(&kvm->arch.iam_ref_lock);
>>>>> +
>>>>> +	return gib->nisc;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(kvm_s390_gisc_register);
>>>>> +
>>>>> +int kvm_s390_gisc_unregister(struct kvm *kvm, u32 gisc)
>>>>> +{
>>>>> +	int rc = 0;
>>>>> +
>>>>> +	if (!kvm->arch.gib_in_use)
>>>>> +		return -ENODEV;
>>>>> +	if (gisc > MAX_ISC)
>>>>> +		return -ERANGE;
>>>>> +
>>>>> +	spin_lock(&kvm->arch.iam_ref_lock);
>>>>> +	if (kvm->arch.iam_ref_count[gisc] == 0) {
>>>>> +		rc = -EINVAL;
>>>>> +		goto out;
>>>>> +	}
>>>>> +	kvm->arch.iam_ref_count[gisc]--;
>>>>> +	if (kvm->arch.iam_ref_count[gisc] == 0) {
>>>>> +		kvm->arch.iam &= ~(0x80 >> gisc);
>>>>> +		set_iam(kvm->arch.gisa, kvm->arch.iam);
>>>
>>> Any chance of this function failing here? If yes, would there be any
>>> implications?
>>
>> It is the same here.
> 
> I'm not sure that I follow: This is the reverse operation
> (unregistering the gisc). Can we rely on get_ipm() to do any fixup
> later? Is that a problem for the caller?
> 
> Apologies if I sound confused (well, that's because I probably am);
> this is hard to review without access to the hardware specification.

I think nothing will happen because the AP CLR IRQ call (Pierre?)
has already taken offline the last AP device.


> 
>>
>>>    
>>>>> +	}
>>>>> +out:
>>>>> +	spin_unlock(&kvm->arch.iam_ref_lock);
>>>>> +
>>>>> +	return rc;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(kvm_s390_gisc_unregister);
>>>>> +
>>>>>     void kvm_s390_gib_destroy(void)
>>>>>     {
>>>>>     	if (!gib)
>>>>>       
>>>>
>>>>   
>>>    
>>
>