Re: [RFC v2 4/9] VFIO: platform: handler tests whether the IRQ is forwarded

From: Christoffer Dall <christoffer.dall@linaro.org>
To: Eric Auger <eric.auger@linaro.org>
Cc: eric.auger@st.com, marc.zyngier@arm.com,
	linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	alex.williamson@redhat.com, joel.schopp@amd.com,
	kim.phillips@freescale.com, paulus@samba.org, gleb@kernel.org,
	pbonzini@redhat.com, linux-kernel@vger.kernel.org,
	patches@linaro.org, will.deacon@arm.com,
	a.motakis@virtualopensystems.com, a.rigo@virtualopensystems.com,
	john.liuli@huawei.com
Subject: Re: [RFC v2 4/9] VFIO: platform: handler tests whether the IRQ is forwarded
Date: Thu, 11 Sep 2014 19:05:54 +0200	[thread overview]
Message-ID: <20140911170554.GB5535@lvm> (raw)
In-Reply-To: <541160D2.80400@linaro.org>

On Thu, Sep 11, 2014 at 10:44:02AM +0200, Eric Auger wrote:
> On 09/11/2014 05:10 AM, Christoffer Dall wrote:
> > On Mon, Sep 01, 2014 at 02:52:43PM +0200, Eric Auger wrote:
> >> In case the IRQ is forwarded, the VFIO platform IRQ handler does not
> >> need to disable the IRQ anymore. In that mode, when the handler completes
> > 
> > add a comma after completes
> Hi Christoffer,
> ok
> > 
> >> the IRQ is not deactivated but only its priority is lowered.
> >>
> >> Some other actor (typically a guest) is supposed to deactivate the IRQ,
> >> allowing at that time a new physical IRQ to hit.
> >>
> >> In virtualization use case, the physical IRQ is automatically completed
> >> by the interrupt controller when the guest completes the corresponding
> >> virtual IRQ.
> >>
> >> Signed-off-by: Eric Auger <eric.auger@linaro.org>
> >> ---
> >>  drivers/vfio/platform/vfio_platform_irq.c | 7 ++++++-
> >>  1 file changed, 6 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/vfio/platform/vfio_platform_irq.c b/drivers/vfio/platform/vfio_platform_irq.c
> >> index 6768508..1f851b2 100644
> >> --- a/drivers/vfio/platform/vfio_platform_irq.c
> >> +++ b/drivers/vfio/platform/vfio_platform_irq.c
> >> @@ -88,13 +88,18 @@ static irqreturn_t vfio_irq_handler(int irq, void *dev_id)
> >>  	struct vfio_platform_irq *irq_ctx = dev_id;
> >>  	unsigned long flags;
> >>  	int ret = IRQ_NONE;
> >> +	struct irq_data *d;
> >> +	bool is_forwarded;
> >>  
> >>  	spin_lock_irqsave(&irq_ctx->lock, flags);
> >>  
> >>  	if (!irq_ctx->masked) {
> >>  		ret = IRQ_HANDLED;
> >> +		d = irq_get_irq_data(irq_ctx->hwirq);
> >> +		is_forwarded = irqd_irq_forwarded(d);
> >>  
> >> -		if (irq_ctx->flags & VFIO_IRQ_INFO_AUTOMASKED) {
> >> +		if (irq_ctx->flags & VFIO_IRQ_INFO_AUTOMASKED &&
> >> +						!is_forwarded) {
> >>  			disable_irq_nosync(irq_ctx->hwirq);
> >>  			irq_ctx->masked = true;
> >>  		}
> >> -- 
> >> 1.9.1
> >>
> > It makes sense that these needs to be all controlled in the kernel, but
> > I'm wondering if it would be cleaner / more correct to clear the
> > AUTOMASKED flag when the IRQ is forwarded and have vfio refuse setting
> > this flag as long as the irq is forwarded?
> 
> If I am not wrong, even if the user sets AUTOMASKED, this info never is
> exploited by the vfio platform driver. AUTOMASKED only is set internally
> to the driver, on init, for level sensitive IRQs.
> 
> It seems to be the same on PCI (for INTx). I do not see anywhere the
> user flag curectly copied into a local storage. But I prefer to be
> careful ;-)
> 
> If confirmed, although the flag value is exposed in the user API, the
> user set value never is exploited so this removes the need to check.
> 
> the forwarded IRQ modality being fully dynamic currently, then I would
> need to update the irq_ctx->flags on each vfio_irq_handler call. I don't
> know if its better?
> 
I'm not an expert on vfio, so I'll leave that to Alex Williamson to
answer, but I'm just worried that we need to special-case the forwarded
IRQ here, and if that may get lost elsewhere in the vfio code.  If the
AUTOMASKED flag covers specifically this behavior, then why don't we
simply clear/set that flag when forwarding/unforwarding the specific
IRQ?

-Christoffer