Re: [RFC PATCH v2 11/19] genirq: Document vcpu_info usage for per-CPU interrupts

Christoffer Dall
2017-08-01T16:57:44Z

On Tue, Aug 01, 2017 at 05:15:39PM +0100, Marc Zyngier wrote:
> On 17/07/17 15:27, Christoffer Dall wrote:
> > It is currently unclear how to set the VCPU affinity for an interrupt
> > which is of the per-CPU kind, since the Linux irq_data structure
>
> very minor nit (which I should have noticed before, my bad): we have two
> kinds of per-CPU interrupts in the kernel. percpu, and percpu_devid. For
> the percpu variety, the IRQ number represents a single interrupt line,
> delivered to a given CPU (and non migrate-able). The percpu_devid
> variant represents multiple interrupts, each assigned to one CPU.
>
> It'd be nice if the commit message (and the comment below) reflected
> this distinction.
>
> > describes the state for multiple interrupts when the interrupt is a
> > per-CPU interrupt, one for each physical CPU on the system. Since each
> > such interrupt can be associated with different VCPUs or none at all,
> > associating a single VCPU state with such an interrupt does not capture
> > the necessary semantics.
> >
> > The implementers of irq_set_affinity are the Intel and AMD IOMMUs, and
> > the ARM GIC irqchip. The Intel and AMD callers do not appear to use
> > per-CPU interrupts, and the ARM GIC implementation only checks the
> > pointer against NULL vs. non-NULL.
> >
> > Therefore, simply update the function documentation to explain the
> > expected use in the context of per-CPU interrupts, allowing future
> > changes or additions to irqchip implementers to do the right thing.
> >
> > This allows us to set the VCPU affinity for the virtual timer interrupt
> > in KVM/ARM, which is a per-CPU (PPI) interrupt.
> >
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Marc Zyngier <marc.zyngier@arm.com>
> > Signed-off-by: Christoffer Dall <cdall@linaro.org>
> > ---
> > kernel/irq/manage.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> > index 5624b2d..050b9f6 100644
> > --- a/kernel/irq/manage.c
> > +++ b/kernel/irq/manage.c
> > @@ -381,7 +381,8 @@ int irq_select_affinity_usr(unsigned int irq)
> > /**
> > * irq_set_vcpu_affinity - Set vcpu affinity for the interrupt
> > * @irq: interrupt number to set affinity
> > - * @vcpu_info: vCPU specific data
> > + * @vcpu_info: vCPU specific data or pointer to a percpu array of vCPU
> > + * specific data for per-CPU interrupts
> > *
> > * This function uses the vCPU specific data to set the vCPU
> > * affinity for an irq. The vCPU specific data is passed from
> >
>
> Otherwise a worthy clarification of the feature.
>
Thanks, I'll try to clarify based on the info below.

Linux IRQ abstractions; I know nothing.

-Christoffer