Re: [PATCH v5 2/2] KVM: Make KVM_CAP_IRQFD dependent on KVM_CAP_IRQCHIP

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Mon, 30 Nov 2015 14:56:38 +0300
Pavel Fedin <p.fedin@samsung.com> wrote:

>  Hello!
> 
> > >  	case KVM_CAP_INTERNAL_ERROR_DATA:
> > >  #ifdef CONFIG_HAVE_KVM_MSI
> > >  	case KVM_CAP_SIGNAL_MSI:
> > > +		/* Fallthrough */
> > >  #endif
> > > +	case KVM_CAP_CHECK_EXTENSION_VM:
> > > +		return 1;
> > >  #ifdef CONFIG_HAVE_KVM_IRQFD
> > >  	case KVM_CAP_IRQFD:
> > >  	case KVM_CAP_IRQFD_RESAMPLE:
> > > +		return kvm_vm_ioctl_check_extension(kvm, KVM_CAP_IRQCHIP);
> > 
> > This won't work for s390, as it doesn't have KVM_CAP_IRQCHIP but
> > KVM_CAP_S390_IRQCHIP (which needs to be enabled).
> 
>  Thank you for the note, i didn't know about irqchip-specific capability codes. There's the same issue with PowerPC, now i
> understand why there's no KVM_CAP_IRQCHIP for them. Because they have KVM_CAP_IRQ_MPIC and KVM_CAP_IRQ_XICS, similar to S390.
>  But isn't it just weird? I understand that perhaps we have some real need to distinguish between different irqchip types, but
> shouldn't the kernel also publish KVM_CAP_IRQCHIP, which stands just for "we support some irqchip virtualization"?
>  May be we should just add this for PowerPC and S390, to make things less ambiguous?

Note that we explicitly need to _enable_ the s390 cap (for
compatibility). I'd need to recall the exact details but I came to the
conclusion back than that I could not simply enable KVM_CAP_IRQCHIP for
s390 (and current qemu would fail to enable the s390 cap if we started
advertising KVM_CAP_IRQCHIP now).