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

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2014-09-11 at 19:05 +0200, Christoffer Dall wrote:
> 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?

The way that VFIO_IRQ_INFO_AUTOMASKED is being used here is unique to
the platform device vfio backend.  In the rest of VFIO,
VFIO_IRQ_INFO_AUTOMASKED is simply a flag bit exposed via
VFIO_DEVICE_GET_IRQ_INFO.  The flags field of struct vfio_irq_info is
output-only.  vfio-pci knows by the IRQ index whether it is edge or
level.  I do agree though that changing the flag bit, or better yet a
bool, rather than adding extra tests that need to be handled as each
usage seems less error prone.

Things could get confusing for userspace though if suddenly
VFIO_DEVICE_GET_IRQ_INFO starts calling the index edge triggered once
forwarding mode is enabled.  Thanks,

Alex