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

[Chao Gao <chao.gao@intel.com>]

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);

can this variable be declared as static?

>> +
>>  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);

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);
}