Re: [Xen-devel] [PATCH for-next] xen/arm: irq: Don't use _IRQ_PENDING when handling host interrupt

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 16 Apr 2019, Julien Grall wrote:
> Hi Stefano,
> 
> On 4/16/19 10:51 PM, Stefano Stabellini wrote:
> > On Mon, 28 Jan 2019, Julien Grall wrote:
> > > While SPIs are shared between CPU, it is not possible to receive the
> > > same interrupts on a different CPU while the interrupt is in active
> > > state. The deactivation of the interrupt is done at the end of the
> > > handling.
> > > 
> > > This means the _IRQ_PENDING logic is unecessary on Arm as a same
> > > interrupt can never come up while in the loop. So remove it to
> > > simplify the interrupt handle code.
> > > 
> > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > > ---
> > >   xen/arch/arm/irq.c | 32 ++++++++++----------------------
> > >   1 file changed, 10 insertions(+), 22 deletions(-)
> > > 
> > > diff --git a/xen/arch/arm/irq.c b/xen/arch/arm/irq.c
> > > index c51cf333ce..3877657a52 100644
> > > --- a/xen/arch/arm/irq.c
> > > +++ b/xen/arch/arm/irq.c
> > > @@ -199,6 +199,7 @@ int request_irq(unsigned int irq, unsigned int
> > > irqflags,
> > >   void do_IRQ(struct cpu_user_regs *regs, unsigned int irq, int is_fiq)
> > >   {
> > >       struct irq_desc *desc = irq_to_desc(irq);
> > > +    struct irqaction *action;
> > >         perfc_incr(irqs);
> > >   @@ -242,35 +243,22 @@ void do_IRQ(struct cpu_user_regs *regs, unsigned
> > > int irq, int is_fiq)
> > >           goto out_no_end;
> > >       }
> > >   -    set_bit(_IRQ_PENDING, &desc->status);
> > > -
> > > -    /*
> > > -     * Since we set PENDING, if another processor is handling a different
> > > -     * instance of this same irq, the other processor will take care of
> > > it.
> > > -     */
> > > -    if ( test_bit(_IRQ_DISABLED, &desc->status) ||
> > > -         test_bit(_IRQ_INPROGRESS, &desc->status) )
> > > +    if ( test_bit(_IRQ_DISABLED, &desc->status) )
> > >           goto out;
> > 
> > It is a good idea to remove the IRQ_PENDING logic, that is OK.
> > 
> > 
> > However, are we sure that we want to remove the _IRQ_INPROGRESS check
> > too? IRQ handlers shouldn't be called twice in a row. Given that
> > _IRQ_INPROGRESS can be set manually (gicv2_set_active_state) it seems it
> > would be a good idea to keep the check anyway?
> 
> set_active_state is only used by the vGIC to replicate state from of the
> virtual interrupt to the physical interrupt. We don't have the virtual
> interrupt in this path (see above).
> 
> Any other user (e.g interrupts routed to Xen) would be pretty broken. At best
> you would break the interrupt flow. At worst, you may never receive the
> interrupt again.
> 
> So I think we can drop _IRQ_PROGRESS here.

I gave it a close look. You are right, it is safe to remove the
_IRQ_PROGRESS check here.

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


The thing that worries me a bit is that technically set_active_state is
part of the gic_hw_operations functions which are not necessarily guest
specific: we haven't written down anywhere that set_active_state cannot
be called passing one of the xen irqs as parameter. I agree it would be
broken to do so, but still... Maybe we should add a comment?

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