Re: [PATCH 2/4] KVM: selftests: Add helper macros for ioctl()s that return file descriptors

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 04, 2023, Colton Lewis wrote:
> Oliver Upton <oliver.upton@linux.dev> writes:
> 
> > Hi Sean,
> 
> > On Thu, Aug 03, 2023 at 05:42:24PM -0700, Sean Christopherson wrote:
> > > Add KVM, VM, and vCPU scoped helpers for ioctl()s that return file
> > > descriptors, i.e. deduplicate code for asserting success on ioctls() for
> > > which a positive return value, not just zero, is considered success.
> 
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> 
> > I appreciate the desire to eliminate duplicate code, but I think the
> > naming just muddies the waters. TBH, when I first read the diff w/o the
> > changelog, I thought you were describing the input fd (i.e. 'kvm_fd',
> > 'vm_fd', 'vcpu_fd'). I don't think explicitly spelling out the condition
> > each time (i.e. ret >= 0) is all that difficult.
> 
> Couldn't ret >= 0 be the assert condition for everything? Don't see why
> there needs to be different helpers to check == 0 and >= 0.
> 
> Unless I'm missing something, error returns are only ever negative.

Using "ret >= 0" would work in the sense that the tests wouldn't fail, but it
would degrade our test coverage, e.g. selftests wouldn't detect KVM bugs where
an ioctl() unexpectedly returns a non-zero, positive value.

The other wrinkle is that selftests need to actually consume the return value for
ioctl()s that return a positive value, i.e. the fd (or whatever it is) needs to
propagated up the stack.  I.e. all of the generic ioctl() macros would need to
"return" the value, which I really don't want to do because that would (re)open
the gates for having helpers that return an int, even though the only possible
return value is '0'.