Re: [PATCH v2 3/3] KVM: x86: Add a module param to control and enumerate device posted IRQs

[Sean Christopherson <seanjc@google.com>]

On Fri, Mar 21, 2025, Chao Gao wrote:
> On Thu, Mar 20, 2025 at 10:59:19AM -0700, Sean Christopherson wrote:
> >@@ -9776,8 +9777,8 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> >        if (r != 0)
> >                goto out_mmu_exit;
> > 
> >-       enable_device_posted_irqs &= enable_apicv &&
> >-                                    irq_remapping_cap(IRQ_POSTING_CAP);
> >+       enable_device_posted_irqs = allow_device_posted_irqs && enable_apicv &&
> >+                                   irq_remapping_cap(IRQ_POSTING_CAP);
> 
> Can we simply drop this ...
> 
> > 
> >        kvm_ops_update(ops);
> > 
> >@@ -14033,6 +14034,8 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_rmp_fault);
> > 
> > static int __init kvm_x86_init(void)
> > {
> >+       allow_device_posted_irqs = enable_device_posted_irqs;
> >+
> >        kvm_init_xstate_sizes();
> > 
> >        kvm_mmu_x86_module_init();
> >
> >
> >Option #2 is to shove the module param into vendor code, but leave the variable
> >in kvm.ko, like we do for enable_apicv.
> >
> >I'm leaning toward option #2, as it's more flexible, arguably more intuitive, and
> >doesn't prevent putting the logic in kvm_x86_vendor_init().
> >
> 
> and do
> 
> bool kvm_arch_has_irq_bypass(void)
> {
> 	return enable_device_posted_irqs && enable_apicv &&
> 	       irq_remapping_cap(IRQ_POSTING_CAP);
> }

That would avoid the vendor module issues, but it would result in
allow_device_posted_irqs not reflecting the state of KVM.  We could partially
address that by having the variable incorporate irq_remapping_cap(IRQ_POSTING_CAP)
but not enable_apicv, but that's still a bit funky.

Given that enable_apicv already has the "variable in kvm.ko, module param in
kvm-{amd,intel}.ko" behavior, and that I am planning on giving enable_ipiv the
same treatment (long story), my strong vote is to go with option #2 as it's the
most flexibile, most accurate, and consistent with existing knobs.