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

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Thu, Mar 20, 2025 at 10:59:19AM -0700, Sean Christopherson wrote:
> On Thu, Mar 20, 2025, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index f76d655dc9a8..e7eb2198db26 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -227,6 +227,10 @@ EXPORT_SYMBOL_GPL(allow_smaller_maxphyaddr);
> >  bool __read_mostly enable_apicv = true;
> >  EXPORT_SYMBOL_GPL(enable_apicv);
> >  
> > +bool __read_mostly enable_device_posted_irqs = true;
> > +module_param(enable_device_posted_irqs, bool, 0444);
> > +EXPORT_SYMBOL_GPL(enable_device_posted_irqs);
> > +
> >  const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
> >  	KVM_GENERIC_VM_STATS(),
> >  	STATS_DESC_COUNTER(VM, mmu_shadow_zapped),
> > @@ -9772,6 +9776,9 @@ 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);
> 
> Drat, this is flawed.  Putting the module param in kvm.ko means that loading
> kvm.ko with enable_device_posted_irqs=true, but a vendor module with APICv/AVIC
> disabled, leaves enable_device_posted_irqs disabled for the lifetime of kvm.ko.
> I.e. reloading the vendor module with APICv/AVIC enabled can't enable device
> posted IRQs.
> 
> Option #1 is to do what we do for enable_mmio_caching, and snapshot userspace's
> desire.
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e7eb2198db26..c84ad9109108 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -228,6 +228,7 @@ bool __read_mostly enable_apicv = true;
>  EXPORT_SYMBOL_GPL(enable_apicv);
>  
>  bool __read_mostly enable_device_posted_irqs = true;
> +bool __ro_after_init allow_device_posted_irqs;
>  module_param(enable_device_posted_irqs, bool, 0444);
>  EXPORT_SYMBOL_GPL(enable_device_posted_irqs);
>  
> @@ -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);
>  
>         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().

+1, option #1 seems a bit confusing to me.