Re: [PATCH for-4.19 4/9] x86/irq: describe how the interrupt CPU movement works

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.05.2024 11:01, Roger Pau Monne wrote:
> --- a/xen/arch/x86/include/asm/irq.h
> +++ b/xen/arch/x86/include/asm/irq.h
> @@ -28,6 +28,32 @@ typedef struct {
>  
>  struct irq_desc;
>  
> +/*
> + * Xen logic for moving interrupts around CPUs allows manipulating interrupts
> + * that target remote CPUs.  The logic to move an interrupt from CPU(s) is as
> + * follows:
> + *
> + * 1. cpu_mask and vector is copied to old_cpu_mask and old_vector.
> + * 2. New cpu_mask and vector are set, vector is setup at the new 
> destination.
> + * 3. move_in_progress is set.
> + * 4. Interrupt source is updated to target new CPU and vector.
> + * 5. Interrupts arriving at old_cpu_mask are processed normally.
> + * 6. When an interrupt is delivered at the new destination (cpu_mask) as 
> part
> + *    of acking the interrupt move_in_progress is cleared and 
> move_cleanup_count

Nit: A comma after "interrupt" may help reading.

> + *    is set to the weight of online CPUs in old_cpu_mask.
> + *    IRQ_MOVE_CLEANUP_VECTOR is sent to all CPUs in old_cpu_mask.

These last two steps aren't precise enough, compared to what the code does.
old_cpu_mask is first reduced to online CPUs therein. If the result is non-
empty, what you describe is done. If, however, the result is empty, the
vector is released right away (this code may be there just in case, but I
think it shouldn't be omitted here).

> + * 7. When receiving IRQ_MOVE_CLEANUP_VECTOR CPUs in old_cpu_mask clean the
> + *    vector entry and decrease the count in move_cleanup_count.  The CPU 
> that
> + *    sets move_cleanup_count to 0 releases the vector.
> + *
> + * Note that when interrupt movement (either move_in_progress or
> + * move_cleanup_count set) is in progress it's not possible to move the
> + * interrupt to yet a different CPU.
> + *
> + * By keeping the vector in the old CPU(s) configured until the interrupt is
> + * acked on the new destination Xen allows draining any pending interrupts at
> + * the old destinations.
> + */
>  struct arch_irq_desc {
>          s16 vector;                  /* vector itself is only 8 bits, */
>          s16 old_vector;              /* but we use -1 for unassigned  */

I take it that it is not a goal to (also) describe under what conditions
an IRQ move may actually be initiated (IRQ_MOVE_PENDING)? I ask not the
least because the 2nd from last paragraph lightly touches that area.

Jan