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

[David Woodhouse]

On Wed, 2026-05-06 at 23:36 -0300, Mauricio Faria de Oliveira wrote:
> kvm_xen_set_evtchn_fast() calls read_lock_irqsave(), which might block
> on PREEMPT_RT, but that is invalid in IRQ context, as when it's called
> by xen_timer_callback() (even on PREEMPT_RT per HRTIMER_MODE_ABS_HARD).
> 
> Check for that case, and bail out early.
> 
> Note: there is previous work and discussion on this [1] (~2 years ago),
> which involved continuing to execute the function with changes, but it
> was not merged. That was a different, more complex approach.
> 
> [1] https://lore.kernel.org/lkml/ZdPQVP7eejq3eFjc@xxxxxxxxxx/

... 

> +	/* Bail in IRQ context on PREEMPT_RT; read_lock_irqsave() might block */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && in_hardirq())
> +		goto out;


The approach in Paul's earlier patch was better; we absolutely *want*
to deliver the interrupt to the guest immediately whenever we can, and
only fall back to the workqueue in the rare case that the shared info
page has been invalidated.

We should switch to plain read_trylock(), *without* the
local_irq_save(). And since this was the *only* case where the GPC lock
was ever taken under IRQ¹, all the GPC locking can drop the _irq part.

Sean's concern was:

>> 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@xxxxxxxxxxxxx/

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