Re: [PATCH 2/3] x86/irq: don't disable domain MSI IRQ on force unbind

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.01.2021 12:06, Roger Pau Monne wrote:
> When force unbinding a MSI the used IRQ would get added to the domain
> irq-pirq tree as -irq -pirq,

I think it's -pirq at index irq, i.e. I don't think IRQ gets
negated as far as the radix tree goes. info->arch.irq gets a
negative value stored, yes.

> thus preventing the same IRQ to be used by the domain.

Iirc this (answering your post-commit-message question here)
is for cleaning up _after_ the domain, i.e. there's no goal
to allow re-use of this IRQ. The comment ahead of
unmap_domain_pirq() validly says "The pirq should have been
unbound before this call." The only time we can't make
ourselves dependent upon this is when the guest is being
cleaned up. During normal operation I think we actually
_want_ to enforce correct behavior of the guest here.

> It's not clear to me why we add the irq as -irq -pirq to the irq-pirq
> domain tree on forced unbind, as allowing to bind the irq again should
> just work regardless of whether it has been previously forcefully
> unbound?

To continue from the above, see pirq_guest_unbind() where
we have

    if ( desc == NULL )
    {
        irq = -pirq->arch.irq;
        BUG_ON(irq <= 0);
        desc = irq_to_desc(irq);
        spin_lock_irq(&desc->lock);
        clear_domain_irq_pirq(d, irq, pirq);
    }

as the alternative to going the normal path through
__pirq_guest_unbind(). Again iirc that's to cover for the
unbind request arriving after the unmap one (i.e. having
caused the unmap to force-unbind the IRQ).

Jan