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

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 7 Jan 2019 18:38:02 +0100
Michael Mueller <mimu@linux.ibm.com> wrote:

> 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.

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.

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