Re: [PATCH 1/3] KVM: x86: Move kvm_(un)register_irq_mask_notifier() to generic KVM

[Dmytro Maluka <dmy@semihalf.com>]

On 7/28/22 20:46, Sean Christopherson wrote:
> On Fri, Jul 15, 2022, Dmytro Maluka wrote:
>> In preparation for implementing postponing resamplefd event until the
>> interrupt is unmasked, move kvm_(un)register_irq_mask_notifier() from
>> x86 to arch-independent code to make it usable by irqfd.
> 
> This patch needs to move more than just the helpers, e.g. mask_notifier_list
> needs to be in "struct kvm", not "stuct kvm_arch".
> 
> arch/arm64/kvm/../../../virt/kvm/eventfd.c: In function ‘kvm_register_irq_mask_notifier’:
> arch/arm64/kvm/../../../virt/kvm/eventfd.c:528:51: error: ‘struct kvm_arch’ has no member named ‘mask_notifier_list’
>   528 |         hlist_add_head_rcu(&kimn->link, &kvm->arch.mask_notifier_list);
>       |                                                   ^
> make[3]: *** [scripts/Makefile.build:249: arch/arm64/kvm/../../../virt/kvm/eventfd.o] Error 1
> make[3]: *** Waiting for unfinished jobs....
>   AR      kernel/entry/built-in.a

Oops, sorry.

> And kvm_fire_mask_notifiers() should probably be moved as well, otherwise there's
> no point in moving the registration to common code.

Good point, we can move it right away, even though it is not called on
other architectures for now.

> The other option would be to make the generic functions wrappers around arch-specific
> hooks.  But IIRC won't this eventually be needed for other architectures?

Right, I assume we will eventually need it for ARM at least. Not in the
near future though, and at the moment I have no non-x86 hardware on hand
to implement it for other architectures.

Actually I feel a bit uncomfortable with generic irqfd relying on
kvm_register_irq_mask_notifier() which silently has no effect on other
architectures. Maybe it's better to keep
kvm_(un)register_irq_mask_notifier() in the x86 code, and for the
generic code add a weak version which e.g. just prints a warning like
"irq mask notifiers not implemented on this arch". (Or maybe instead of
weak functions introduce arch-specific hooks as you suggested, and print
such a warning if no hook is provided.) What do you think?

Thanks,
Dmytro