Re: [PATCH v2] x86/apic: Avoid infinite loop in io_apic_level_ack_pending()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Oct 13, 2025 at 05:11:06PM -0400, Jason Andryuk wrote:
> io_apic_level_ack_pending() will end up in an infinite loop if
> entry->pin == -1.  entry does not change, so it will keep reading -1.

Do you know how you end up with an entry with pin == -1 on the
irq_pin_list? Are there systems with gaps in the GSI space between
IO-APICs?  So far everything I saw had the IO-APIC in contiguous GSI
space.

> Convert to a proper for loop so that continue works.  Add a new helper,
> next_entry(), to handle advancing to the next irq_pin_list entry.
> 
> Fixes: f821102450a1 ("x86: IRQ Migration logic enhancement.")
> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> ---
> v2:
> continue (not break) for pin == -1.
> 
> I added the next_entry() helper since putting the expression in the for
> loop is a little cluttered.  The helper can also be re-used for other
> instances within the file.
> ---
>  xen/arch/x86/io_apic.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
> index c384f10c1b..7b58345c96 100644
> --- a/xen/arch/x86/io_apic.c
> +++ b/xen/arch/x86/io_apic.c
> @@ -1586,14 +1586,21 @@ static int __init cf_check setup_ioapic_ack(const 
> char *s)
>  }
>  custom_param("ioapic_ack", setup_ioapic_ack);
>  
> +static struct irq_pin_list *next_entry(struct irq_pin_list *entry)

I think you can make the entry parameter const?

> +{
> +    if ( !entry->next )
> +        return NULL;
> +
> +    return irq_2_pin + entry->next;
> +}
> +
>  static bool io_apic_level_ack_pending(unsigned int irq)
>  {
>      struct irq_pin_list *entry;
>      unsigned long flags;
>  
>      spin_lock_irqsave(&ioapic_lock, flags);
> -    entry = &irq_2_pin[irq];
> -    for (;;) {
> +    for ( entry = &irq_2_pin[irq]; entry ; entry = next_entry(entry) ) {

I'm not sure where we stand regarding coding style here, but it looks
you either want to remove the space between parentheses (my
preference), or place the opening for braces on a newline.  I would
possibly do:

for (entry = &irq_2_pin[irq]; entry; entry = next_entry(entry)) {
...

As I think it fits better given the small change and the surrounding
coding style.

>          unsigned int reg;
>          int pin;

Below here you can remove the:

        if (!entry)
            break;

Chunk, as the for loop already checks for this condition.

Otherwise looks good, I think we should consider for 4.21 inclusion.

Thanks, Roger.