Re: [PATCH v6 18/23] xen/riscv: implement IRQ routing for device passthrough

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

On 7/29/26 5:02 PM, Oleksii Kurochko wrote:
> 
> 
> On 7/29/26 4:15 PM, Jan Beulich wrote:
>> On 29.07.2026 13:59, Oleksii Kurochko wrote:
>>> On 7/23/26 3:30 PM, Jan Beulich wrote:
>>>> On 20.07.2026 17:59, Oleksii Kurochko wrote:
>>>>> +/* Route an IRQ to a specific guest */
>>>>> +int route_irq_to_guest(struct domain *d, unsigned int virq,
>>>>> +                       unsigned int irq, const char *devname)
>>>>> +{
>>>>> +    struct irqaction *action;
>>>>> +    struct irq_guest *info;
>>>>> +    struct irq_desc *desc;
>>>>> +    unsigned long flags;
>>>>> +    int retval = 0;
>>>>> +
>>>>> +    if ( d->is_dying )
>>>>> +        return -EINVAL;
>>>>> +
>>>>> +    desc = irq_to_desc(irq);
>>>>> +
>>>>> +    /*
>>>>> +     * release_irq() frees this action via xvfree(), relying on 
>>>>> action
>>>>> +     * being the first member of struct irq_guest so that &info- 
>>>>> >action
>>>>> +     * coincides with info itself. Guard the layout so a future field
>>>>> +     * reorder can't silently turn that into a free() of a mid- 
>>>>> allocation
>>>>> +     * pointer.
>>>>> +     */
>>>>> +    BUILD_BUG_ON(offsetof(struct irq_guest, action) != 0);
>>>>
>>>> Can't release_irq() simply use container_of()? One way or another it 
>>>> feels
>>>> like you're painting yourself into a particular corner ...
>>>
>>> If it isn't the best option then it is needed to follow they way we had
>>> before:
>>
>> I don't understand why you think you need to go back.
> 
> Because, based on your reply—specifically, "One way or another it feels 
> like you're painting yourself into a particular corner..." — it seems 
> that even if I replaced BUILD_BUG_ON() with container_of() in 
> release_irq(), you would still consider it a bad solution. Did I 
> misunderstand your point?

One more thing: I'm not really sure it's safe to do the following in 
release_irq():

if ( action->free_on_release )
     xvfree(container_of(action, struct irq_guest, action));

release_irq() is a generic API, but this kind of allocation is only 
needed for guest IRQs. Wouldn't it be better to set:

action->free_on_release = false;

for guest IRQs, and then free the memory in release_guest_irq() after 
the call to release_irq()?

Wouldn't that be a better approach than calling 
`xvfree(container_of(...))` from within the generic release_irq()?


~ Oleksii