linux-kernel - Re: [PATCH v3 04/10] VFIO: platform: add vfio_platform_set

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Message-ID: <1439919864.4023.626.camel@redhat.com>
Date:	Tue, 18 Aug 2015 11:44:24 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Eric Auger <eric.auger@...aro.org>
Cc:	eric.auger@...com, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
	christoffer.dall@...aro.org, marc.zyngier@....com,
	feng.wu@...el.com, linux-kernel@...r.kernel.org,
	patches@...aro.org, pbonzini@...hat.com
Subject: Re: [PATCH v3 04/10] VFIO: platform: add
 vfio_platform_set_automasked

On Mon, 2015-08-17 at 17:38 +0200, Eric Auger wrote:
> On 08/12/2015 08:56 PM, Alex Williamson wrote:
> > On Mon, 2015-08-10 at 15:20 +0200, Eric Auger wrote:
> >> This function makes possible to change the automasked mode.
> >>
> >> Signed-off-by: Eric Auger <eric.auger@...aro.org>
> >>
> >> ---
> >>
> >> v1 -> v2:
> >> - set forwarded flag
> >> ---
> >>  drivers/vfio/platform/vfio_platform_irq.c | 19 +++++++++++++++++++
> >>  1 file changed, 19 insertions(+)
> >>
> >> diff --git a/drivers/vfio/platform/vfio_platform_irq.c b/drivers/vfio/platform/vfio_platform_irq.c
> >> index b31b1f0..a285384 100644
> >> --- a/drivers/vfio/platform/vfio_platform_irq.c
> >> +++ b/drivers/vfio/platform/vfio_platform_irq.c
> >> @@ -186,6 +186,25 @@ static irqreturn_t vfio_handler(int irq, void *dev_id)
> >>  	return ret;
> >>  }
> >>  
> >> +static int vfio_platform_set_automasked(struct vfio_platform_irq *irq,
> >> +					   bool automasked)
> >> +{
> >> +	unsigned long flags;
> >> +
> >> +	spin_lock_irqsave(&irq->lock, flags);
> >> +	if (automasked) {
> >> +		irq->forwarded = true;
> >> +		irq->flags |= VFIO_IRQ_INFO_AUTOMASKED;
> >> +		irq->handler = vfio_automasked_irq_handler;
> >> +	} else {
> >> +		irq->forwarded = false;
> >> +		irq->flags &= ~VFIO_IRQ_INFO_AUTOMASKED;
> >> +		irq->handler = vfio_irq_handler;
> >> +	}
> >> +	spin_unlock_irqrestore(&irq->lock, flags);
> >> +	return 0;
> > 
> > In vfio-speak, automasked means level and we're not magically changing
> > the IRQ from level to edge, we're simply able to handle level
> > differently based on a hardware optimization.  Should the user visible
> > flags therefore change based on this?  Aren't we really setting the
> > forwarded state rather than the automasked state?
> 
> Well actually this was following the discussion we had a long time ago
> about that topic:
> 
> http://lkml.iu.edu/hypermail/linux/kernel/1409.1/03659.html
> 
> I did not really know how to conclude ...
> 
> If it is preferred I can hide this to the userspace, no problem.

I think that was based on the user being involved in enabling forwarding
though, now that it's hidden and automatic, it doesn't make much sense
to me to toggle any of the interrupt info details based on the state of
the forward.  The user always needs to handle the interrupt as level
since the bypass can be torn down at any point in time.  We're taking
advantage of the in-kernel path to make further optimizations, which
seems like they should be transparent to the user.  Thanks,

Alex

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