diff mbox

[v4,12/20] genirq: Document vcpu_info usage for percpu_devid interrupts

Message ID 20171020114939.12554-13-christoffer.dall@linaro.org (mailing list archive)
State New, archived
Headers show

Commit Message

Christoffer Dall Oct. 20, 2017, 11:49 a.m. UTC 
From: Christoffer Dall <cdall@linaro.org>

It is currently unclear how to set the VCPU affinity for a percpu_devid
interrupt , since the Linux irq_data structure describes the state for
multiple interrupts, 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
percpu_devid 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 percpu_devid 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 percpu_devid (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(-)

Comments

Thomas Gleixner Oct. 20, 2017, 11:56 a.m. UTC | #1 
On Fri, 20 Oct 2017, Christoffer Dall wrote:

Please Cc lkml when changing genirq core code.

> It is currently unclear how to set the VCPU affinity for a percpu_devid
> interrupt , since the Linux irq_data structure describes the state for
> multiple interrupts, 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
> percpu_devid 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 percpu_devid 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 percpu_devid (PPI) interrupt.

I have a hard time to understand how adding the comment allows you to set
VCPU affinity ....

Thanks,

	tglx
Christoffer Dall Oct. 21, 2017, 2:29 p.m. UTC | #2 
On Fri, Oct 20, 2017 at 01:56:16PM +0200, Thomas Gleixner wrote:
> On Fri, 20 Oct 2017, Christoffer Dall wrote:
> 
> Please Cc lkml when changing genirq core code.
> 

ok, will do so in the future.

> > It is currently unclear how to set the VCPU affinity for a percpu_devid
> > interrupt , since the Linux irq_data structure describes the state for
> > multiple interrupts, 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
> > percpu_devid 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 percpu_devid 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 percpu_devid (PPI) interrupt.
> 
> I have a hard time to understand how adding the comment allows you to set
> VCPU affinity ....
> 

Obviously everything already works without adding the comment, however
it is not entirely clear how to use the vcpu_into for percpu_devid
interrupts, and this patch aims to make that clear, as it is the first
user of irq_set_vcpu_affinity for a percpu_devid interrupt which
actually uses the vcpu_info pointer.

Would you rather that we did not update the documentation or what do you
suggest?

Thanks,
-Christoffer
Thomas Gleixner Oct. 21, 2017, 2:55 p.m. UTC | #3 
On Sat, 21 Oct 2017, Christoffer Dall wrote:

> On Fri, Oct 20, 2017 at 01:56:16PM +0200, Thomas Gleixner wrote:
> > On Fri, 20 Oct 2017, Christoffer Dall wrote:
> > 
> > Please Cc lkml when changing genirq core code.
> > 
> 
> ok, will do so in the future.
> 
> > > It is currently unclear how to set the VCPU affinity for a percpu_devid
> > > interrupt , since the Linux irq_data structure describes the state for
> > > multiple interrupts, 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
> > > percpu_devid 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 percpu_devid 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 percpu_devid (PPI) interrupt.
> > 
> > I have a hard time to understand how adding the comment allows you to set
> > VCPU affinity ....
> > 
> 
> Obviously everything already works without adding the comment, however
> it is not entirely clear how to use the vcpu_into for percpu_devid
> interrupts, and this patch aims to make that clear, as it is the first
> user of irq_set_vcpu_affinity for a percpu_devid interrupt which
> actually uses the vcpu_info pointer.
> 
> Would you rather that we did not update the documentation or what do you
> suggest?

Documentation is welcome of course. I just want a changelog which is not
misleading. i.e. it says:

   Update the function documentation .....

   This allow us to set ....

Which reads like: Updating the documentation allows us to set ...

See?

Thanks,

	tglx
Christoffer Dall Oct. 21, 2017, 3:30 p.m. UTC | #4 
On Sat, Oct 21, 2017 at 04:55:01PM +0200, Thomas Gleixner wrote:
> On Sat, 21 Oct 2017, Christoffer Dall wrote:
> 
> > On Fri, Oct 20, 2017 at 01:56:16PM +0200, Thomas Gleixner wrote:
> > > On Fri, 20 Oct 2017, Christoffer Dall wrote:
> > > 
> > > Please Cc lkml when changing genirq core code.
> > > 
> > 
> > ok, will do so in the future.
> > 
> > > > It is currently unclear how to set the VCPU affinity for a percpu_devid
> > > > interrupt , since the Linux irq_data structure describes the state for
> > > > multiple interrupts, 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
> > > > percpu_devid 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 percpu_devid 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 percpu_devid (PPI) interrupt.
> > > 
> > > I have a hard time to understand how adding the comment allows you to set
> > > VCPU affinity ....
> > > 
> > 
> > Obviously everything already works without adding the comment, however
> > it is not entirely clear how to use the vcpu_into for percpu_devid
> > interrupts, and this patch aims to make that clear, as it is the first
> > user of irq_set_vcpu_affinity for a percpu_devid interrupt which
> > actually uses the vcpu_info pointer.
> > 
> > Would you rather that we did not update the documentation or what do you
> > suggest?
> 
> Documentation is welcome of course. I just want a changelog which is not
> misleading. i.e. it says:
> 
>    Update the function documentation .....
> 
>    This allow us to set ....
> 
> Which reads like: Updating the documentation allows us to set ...

Ah, your comment was regarding the commit message.  That's a fair point.
How about:

  This documents the use of the vcpu_info parameter for percpu_devid
  interrupts when setting the VCPU affinity for the virtual timer interrupt
  in KVM/ARM.


Thanks,
-Christoffer
diff mbox

Patch

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index d00132b5c325..38ec7a83bf56 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 percpu_devid interrupts
  *
  *	This function uses the vCPU specific data to set the vCPU
  *	affinity for an irq. The vCPU specific data is passed from