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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

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