linux-kernel - Re: [RFC v2 8/9] KVM: KVM-VFIO: generic KVM_DEV_VFIO_DEVICE command and IRQ forwarding control

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Date:	Thu, 11 Sep 2014 14:59:53 -0700
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Eric Auger <eric.auger@...aro.org>, eric.auger@...com,
	marc.zyngier@....com, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
	joel.schopp@....com, kim.phillips@...escale.com, paulus@...ba.org,
	gleb@...nel.org, pbonzini@...hat.com, linux-kernel@...r.kernel.org,
	patches@...aro.org, will.deacon@....com,
	a.motakis@...tualopensystems.com, a.rigo@...tualopensystems.com,
	john.liuli@...wei.com
Subject: Re: [RFC v2 8/9] KVM: KVM-VFIO: generic KVM_DEV_VFIO_DEVICE command
 and IRQ forwarding control

On Thu, Sep 11, 2014 at 12:14:10PM -0600, Alex Williamson wrote:
> On Thu, 2014-09-11 at 19:10 +0200, Christoffer Dall wrote:
> > On Wed, Sep 10, 2014 at 11:05:49PM -0600, Alex Williamson wrote:
> > > On Thu, 2014-09-11 at 05:10 +0200, Christoffer Dall wrote:
> > > > On Mon, Sep 01, 2014 at 02:52:47PM +0200, Eric Auger wrote:
> > 
> > [...]
> > 
> > > > >  
> > > > > +#ifdef __KVM_HAVE_ARCH_KVM_VFIO_FORWARD
> > > > > +int kvm_arch_set_fwd_state(struct kvm_fwd_irq *pfwd,
> > > > 
> > > > what's the 'p' in pfwd?
> > > 
> > > p is for pointer?
> > > 
> > 
> > shouldn't the type declation spell out quite clearly to me that I'm
> > dealing with a pointer?
> 
> Sure.  In the cases where I've done similar things it's more a matter of
> not needing to come up with another variable, for instance if I need
> both a struct and a struct* I might call them foo and pfoo if I can't
> come up with anything more meaningful.
> 
> 
> > [...]
> > 
> > > > 
> > > > need some spaceing here, also, I would turn this around, first check if
> > > > the strcmp fails, and then error out, then do you next check etc., to
> > > > avoid so many nested statements.
> > > > 
> > > > > +	/* is a ref to this device already owned by the KVM-VFIO device? */
> > > > 
> > > > this comment is not particularly helpful in its current form, it would
> > > > be helpful if you specified that we're checking whether that particular
> > > > device/irq combo is already registered.
> > > > 
> > > > > +	*kvm_vdev = kvm_vfio_find_device(kv, vdev);
> > > > > +	if (*kvm_vdev) {
> > > > > +		if (kvm_vfio_find_irq(*kvm_vdev, fwd_irq->index)) {
> > > > > +			kvm_err("%s irq %d already forwarded\n",
> > > > > +				__func__, *hwirq);
> > > 
> > > Why didn't we do this first?
> > > 
> > huh?
> 
> The code is doing:
> 
> 1. can the arch forward this irq
> 2. are we already forwarding this irq
> 
> It's backwards, test for duplicates locally before calling out into arch
> code.  Besides, I think the arch code here should go away and just be
> another error condition for the call-out.  Thanks,
> 
Ah, right, you meant for the whole check.  I agree completely.

-Christoffer
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