[kvmarm] [PATCH v2 2/2] ARM: KVM: Power State Coordination Interface implementation

[Russell King - ARM Linux]

On Fri, Jan 11, 2013 at 12:33:15PM -0500, Christoffer Dall wrote:
> On Fri, Jan 11, 2013 at 12:24 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> > On 11/01/13 17:12, Russell King - ARM Linux wrote:
> >> On Thu, Jan 10, 2013 at 04:06:45PM +0000, Marc Zyngier wrote:
> >>> +int kvm_psci_call(struct kvm_vcpu *vcpu)
> >>> +{
> >>> +    unsigned long psci_fn = *vcpu_reg(vcpu, 0) & ~((u32) 0);
> >>> +    unsigned long val;
> >>> +
> >>> +    switch (psci_fn) {
> >>> +    case KVM_PSCI_FN_CPU_OFF:
> >>> +            kvm_psci_vcpu_off(vcpu);
> >>> +            val = KVM_PSCI_RET_SUCCESS;
> >>> +            break;
> >>> +    case KVM_PSCI_FN_CPU_ON:
> >>> +            val = kvm_psci_vcpu_on(vcpu);
> >>> +            break;
> >>> +    case KVM_PSCI_FN_CPU_SUSPEND:
> >>> +    case KVM_PSCI_FN_MIGRATE:
> >>> +            val = KVM_PSCI_RET_NI;
> >>> +            break;
> >>> +
> >>> +    default:
> >>> +            return -1;
> >>> +    }
> >>> +
> >>> +    *vcpu_reg(vcpu, 0) = val;
> >>> +    return 0;
> >>> +}
> >>
> >> We were discussing recently on #kernel about kernel APIs and the way that
> >> our integer-returning functions pretty much use 0 for success, and -errno
> >> for failures, whereas our pointer-returning functions are a mess.
> >>
> >> And above we have something returning -1 to some other chunk of code outside
> >> this compilation unit.  That doesn't sound particularly clever to me.
> >
> > The original code used to return -EINVAL, see:
> > https://lists.cs.columbia.edu/pipermail/kvmarm/2013-January/004509.html
> >
> > Christoffer (Cc-ed) didn't like this, hence the -1. I'm happy to revert
> > the code to its original state though.
> >
> I don't want to return -EINVAL, because for the rest of the KVM code
> this would mean kill the guest.
> 
> The convention used in other archs of KVM as well as for ARM is that
> the handle_exit functions return:
> 
> -ERRNO: Error, report this error to user space
> 0: Everything is fine, but return to user space to let it do I/O
> emulation and whatever it wants to do
> 1: Everything is fine, return directly to the guest without going to user space

Right, so the above "return -1" _is_ doing the thing that I really hate,
which is it's actually doing a "return -EPERM" without anyone realising
that's what it's doing.

This is precisely why I hate (and pick up on, and have my mail reader to
highlight) any "return -1".  It's mostly always a bug.