Re: [dpdk-dev] [EXT] Re: [PATCH v2 2/3] eal: add ack interrupt API

[Nithin Kumar Dabilpuram <ndabilpuram@marvell.com>]

On Wed, Jul 17, 2019 at 03:05:47PM +0000, Hyong Youb Kim (hyonkim) wrote:
> > -----Original Message-----
> > From: Nithin Kumar Dabilpuram <ndabilpuram@marvell.com>
> > Sent: Wednesday, July 17, 2019 11:36 PM
> [...]
> > > > Subject: [PATCH v2 2/3] eal: add ack interrupt API
> > > >
> > > > Add new ack interrupt API to avoid using
> > > > VFIO_IRQ_SET_ACTION_TRIGGER(rte_intr_enable()) for
> > > > acking interrupt purpose for VFIO based interrupt handlers.
> > > > This implementation is specific to Linux.
> > > >
> > > > Using rte_intr_enable() for acking interrupt has below issues
> > > >
> > > >  * Time consuming to do for every interrupt received as it will
> > > >    free_irq() followed by request_irq() and all other initializations
> > > >  * A race condition because of a window between free_irq() and
> > > >    request_irq() with packet reception still on and device still
> > > >    enabled and would throw warning messages like below.
> > > >    [158764.159833] do_IRQ: 9.34 No irq handler for vector
> > > >
> > > > In this patch, rte_intr_ack() is a no-op for VFIO_MSIX/VFIO_MSI
> > interrupts
> > > > as they are edge triggered and kernel would not mask the interrupt
> > before
> > > > delivering the event to userspace and we don't need to ack.
> > > >
> > > > Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
> > > > Signed-off-by: Jerin Jacob <jerinj@marvell.com>
> > > > ---
> > > > v2:
> > > > * No change
> > > >
> > > >  lib/librte_eal/common/include/rte_interrupts.h | 22 +++++++
> > > >  lib/librte_eal/freebsd/eal/eal_interrupts.c    |  9 +++
> > > >  lib/librte_eal/linux/eal/eal_interrupts.c      | 81
> > > > ++++++++++++++++++++++++++
> > > >  lib/librte_eal/rte_eal_version.map             |  1 +
> > > >  4 files changed, 113 insertions(+)
> > > >
> > > > diff --git a/lib/librte_eal/common/include/rte_interrupts.h
> > > > b/lib/librte_eal/common/include/rte_interrupts.h
> > > > index c1e912c..93b31cd 100644
> > > > --- a/lib/librte_eal/common/include/rte_interrupts.h
> > > > +++ b/lib/librte_eal/common/include/rte_interrupts.h
> > > > @@ -118,6 +118,28 @@ int rte_intr_enable(const struct rte_intr_handle
> > > > *intr_handle);
> > > >   */
> > > >  int rte_intr_disable(const struct rte_intr_handle *intr_handle);
> > > >
> > > > +/**
> > > > + * It acks an interrupt raised for the specified handle.
> > > > + *
> > > > + * Call this function to ack an interrupt from interrupt
> > > > + * handler either from application or driver, so that
> > > > + * new interrupts are raised.
> > > > + *
> > > > + * @note For interrupt handle types VFIO_MSIX and VFIO_MSI,
> > > > + *    this function is a no-op and returns success without
> > > > + *    changing anything as kernel doesn't expect
> > > > + *    them to be acked.
> > > > + *
> > > [...]
> > >
> > > Shouldn't we explain that this really is "unmask" but named "ack" because
> > > of x and y, and that it is expected at end of INTx handler? Ack does
> > > not have a well-defined meaning, whereas everyone knows what unmask
> > > means..
> > >
> > 
> > 
> > Ok. Is the below text fine with you ? Or please suggest.
> > 
> > @note For interrupt handle types VFIO_MSIX and VFIO_MSI,
> >    this function is a no-op and returns success without
> >    changing anything as kernel doesn't expect
> >    them to be acked.
> >    This needs be used atleast for PCI devices with INTx interrupt
> >    as kernel before passing on event for INTx triggered interrupt,
> >    masks the interrupt and expects application to unmask it so that,
> >    further interrupts can be raised/triggered. This is also due to
> >    the fact that INTx is level triggered interrupt where as MSI/MSIx
> >    is not. Ideally this should have been called as intr_unmask()
> >    representing underlying api, but since unmask operation
> >    is not supported and not needed for VFIO MSI/MSIx interrupts
> >    after handling, it is named as ack.
> > 
> 
> How about this?
> 
> PMD generally calls this function at the end of its IRQ callback.
> Internally, it unmasks the interrupt if possible. For INTx, unmasking
> is required as the interrupt is auto-masked prior to invoking
> callback. For MSI/MSI-X, unmasking is typically not needed as the
> interrupt is not auto-masked. In fact, for interrupt handle types
> VFIO_MSIX and VFIO_MSI, this function is no-op.
>
Ok. Thanks. Will add this in next revision.

> Thanks for your effort..
> -Hyong
>