Re: [PATCH] KVM: x86/xen: bail in IRQ context on PREEMPT_RT in kvm_xen_set_evtchn_fast()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-05-07 07:58:00 [+0100], David Woodhouse wrote:
> >> I am not comfortable applying this patch.  As shown by the need for the next patch
> >> to optimize unrelated invalidations, switching to read_trylock() is more subtle
> >> than it seems at first glance.  Specifically, there are no fairness guarantees.
> 
> I'm OK with that in this case. Because kvm_xen_set_evtchn_fast(), as
> with *everything* called from kvm_arch_set_irq_inatomic(), is
> explicitly designed to be a 'best effort' and allowed to return
> -EWOULDBLOCK when it's too hard.
> 
> And the write lock being held here should a *rare* case, as the GPC for
> the shared_info and vcpu_info pages should basically *never* get
> invalidated while the guest is running.
> 
> I've taken the same read_trylock() approach in 
> https://lore.kernel.org/all/1d6712ed413ea66ef376d1410811997c3b416e99.camel@infradead.org/

So the cited patch does not look bad. That read-trylock should be fine
on RT (as in I don't see anything wrong with it). What did happen to it?

Sebastian