Re: [PATCH v2 7/7] x86/irq: forward pending interrupts to new destination in fixup_irqs()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.06.2024 16:20, Roger Pau Monne wrote:
> fixup_irqs() is used to evacuate interrupts from to be offlined CPUs.  Given
> the CPU is to become offline, the normal migration logic used by Xen where the
> vector in the previous target(s) is left configured until the interrupt is
> received on the new destination is not suitable.
> 
> Instead attempt to do as much as possible in order to prevent loosing
> interrupts.  If fixup_irqs() is called from the CPU to be offlined (as is
> currently the case) attempt to forward pending vectors when interrupts that
> target the current CPU are migrated to a different destination.
> 
> Additionally, for interrupts that have already been moved from the current CPU
> prior to the call to fixup_irqs() but that haven't been delivered to the new
> destination (iow: interrupts with move_in_progress set and the current CPU set
> in ->arch.old_cpu_mask) also check whether the previous vector is pending and
> forward it to the new destination.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Changes since v1:
>  - Rename to apic_irr_read().
> ---
>  xen/arch/x86/include/asm/apic.h |  5 +++++
>  xen/arch/x86/irq.c              | 37 ++++++++++++++++++++++++++++++++-
>  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/include/asm/apic.h b/xen/arch/x86/include/asm/apic.h
> index d1cb001fb4ab..7bd66dc6e151 100644
> --- a/xen/arch/x86/include/asm/apic.h
> +++ b/xen/arch/x86/include/asm/apic.h
> @@ -132,6 +132,11 @@ static inline bool apic_isr_read(uint8_t vector)
>              (vector & 0x1f)) & 1;
>  }
>  
> +static inline bool apic_irr_read(unsigned int vector)
> +{
> +    return apic_read(APIC_IRR + (vector / 32 * 0x10)) & (1U << (vector % 
> 32));
> +}
> +
>  static inline u32 get_apic_id(void)
>  {
>      u32 id = apic_read(APIC_ID);
> diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
> index 54eabd23995c..ed262fb55f4a 100644
> --- a/xen/arch/x86/irq.c
> +++ b/xen/arch/x86/irq.c
> @@ -2601,7 +2601,7 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
>  
>      for ( irq = 0; irq < nr_irqs; irq++ )
>      {
> -        bool break_affinity = false, set_affinity = true;
> +        bool break_affinity = false, set_affinity = true, check_irr = false;
>          unsigned int vector, cpu = smp_processor_id();
>          cpumask_t *affinity = this_cpu(scratch_cpumask);
>  
> @@ -2649,6 +2649,25 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
>               !cpumask_test_cpu(cpu, &cpu_online_map) &&
>               cpumask_test_cpu(cpu, desc->arch.old_cpu_mask) )
>          {
> +            /*
> +             * This to be offlined CPU was the target of an interrupt that's
> +             * been moved, and the new destination target hasn't yet
> +             * acknowledged any interrupt from it.
> +             *
> +             * We know the interrupt is configured to target the new CPU at
> +             * this point, so we can check IRR for any pending vectors and
> +             * forward them to the new destination.
> +             *
> +             * Note the difference between move_in_progress or
> +             * move_cleanup_count being set.  For the later we know the new
> +             * destination has already acked at least one interrupt from this
> +             * source, and hence there's no need to forward any stale
> +             * interrupts.
> +             */

I'm a little confused by this last paragraph: It talks about a difference,
yet ...

> +            if ( apic_irr_read(desc->arch.old_vector) )
> +                send_IPI_mask(cpumask_of(cpumask_any(desc->arch.cpu_mask)),
> +                              desc->arch.vector);

... in the code being commented there's no difference visible. Hmm, I guess
this is related to the enclosing if(). Maybe this could be worded a little
differently, e.g. starting with "Note that for the other case -
move_cleanup_count being non-zero - we know ..."?

> @@ -2689,11 +2708,27 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
>          if ( desc->handler->disable )
>              desc->handler->disable(desc);
>  
> +        /*
> +         * If the current CPU is going offline and is (one of) the target(s) 
> of
> +         * the interrupt signal to check whether there are any pending 
> vectors
> +         * to be handled in the local APIC after the interrupt has been 
> moved.
> +         */

After reading this a number of times, I think there wants to be a comma between
"interrupt" and "signal". Or am I getting wrong what is being meant?

> +        if ( !cpu_online(cpu) && cpumask_test_cpu(cpu, desc->arch.cpu_mask) )
> +            check_irr = true;
> +
>          if ( desc->handler->set_affinity )
>              desc->handler->set_affinity(desc, affinity);
>          else if ( !(warned++) )
>              set_affinity = false;
>  
> +        if ( check_irr && apic_irr_read(vector) )
> +            /*
> +             * Forward pending interrupt to the new destination, this CPU is
> +             * going offline and otherwise the interrupt would be lost.
> +             */
> +            send_IPI_mask(cpumask_of(cpumask_any(desc->arch.cpu_mask)),
> +                          desc->arch.vector);
> +
>          if ( desc->handler->enable )
>              desc->handler->enable(desc);
>  

Down from here, after the loop, there's a 1ms window where latched but not
yet delivered interrupts can be received. How's that playing together with
the changes you're making? Aren't we then liable to get two interrupts, one
at the old and one at the new source, in unknown order?

Jan