Re: [PATCH for-4.19 v4] x86/irq: forward pending interrupts to new destination in fixup_irqs()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.06.2024 11:58, 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 for CPU hot unplug) 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.
> 
> This allows us to remove the window with interrupts enabled at the bottom of
> fixup_irqs().  Such window wasn't safe anyway: references to the CPU to become
> offline are removed from interrupts masks, but the per-CPU vector_irq[] array
> is not updated to reflect those changes (as the CPU is going offline anyway).
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>