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

[Michael Mueller <mimu@linux.ibm.com>]

On 04.01.19 14:19, Cornelia Huck wrote:
> On Wed, 2 Jan 2019 18:29:00 +0100
> Pierre Morel <pmorel@linux.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@linux.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.

> 
>>> +	}
>>> +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)
>>>    
>>
>>
>