Re: [PATCH v1] KVM: x86: add KVM_CAP_DEVICE_CTRL

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Thu, Dec 15, 2022, Wei Wang wrote:
> KVM_CAP_DEVICE_CTRL allows userspace to create emulated device in KVM.
> For example, userspace VFIO implementation needs to create a kvm_device
> (i.e. KVM_DEV_TYPE_VFIO) on x86. So add the cap to allow userspace for
> such use cases.
> 
> Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 69227f77b201..1cdc4469652c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4410,6 +4410,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_VAPIC:
>  	case KVM_CAP_ENABLE_CAP:
>  	case KVM_CAP_VM_DISABLE_NX_HUGE_PAGES:
> +	case KVM_CAP_DEVICE_CTRL:

Rather than hardcode this in x86, I think it would be better to add an #ifdef'd
version in the generic check.  E.g. if MIPS or RISC-V ever gains KVM_VFIO support
then they'll need to enumerate KVM_CAP_DEVICE_CTRL too, and odds are we'll forget
to to do.

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 13e88297f999..f70b9cea95d9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4525,6 +4525,10 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
        case KVM_CAP_BINARY_STATS_FD:
        case KVM_CAP_SYSTEM_EVENT_DATA:
                return 1;
+#ifdef CONFIG_KVM_VFIO
+       case KVM_CAP_DEVICE_CTRL:
+               return 1;
+#endif
        default:
                break;
        }

The other potentially bad idea would be to detect the presence of a device_ops and
delete all of the arch hooks, e.g. 

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 9c5573bc4614..190e9c3b10a7 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -212,7 +212,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
                r = vgic_present;
                break;
        case KVM_CAP_IOEVENTFD:
-       case KVM_CAP_DEVICE_CTRL:
        case KVM_CAP_USER_MEMORY:
        case KVM_CAP_SYNC_MMU:
        case KVM_CAP_DESTROY_MEMORY_REGION_WORKS:
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index 04494a4fb37a..21f9fbe96f6a 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -541,7 +541,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
        case KVM_CAP_ENABLE_CAP:
        case KVM_CAP_ONE_REG:
        case KVM_CAP_IOEVENTFD:
-       case KVM_CAP_DEVICE_CTRL:
        case KVM_CAP_IMMEDIATE_EXIT:
        case KVM_CAP_SET_GUEST_DEBUG:
                r = 1;
diff --git a/arch/riscv/kvm/vm.c b/arch/riscv/kvm/vm.c
index 65a964d7e70d..6efe93b282e1 100644
--- a/arch/riscv/kvm/vm.c
+++ b/arch/riscv/kvm/vm.c
@@ -57,7 +57,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 
        switch (ext) {
        case KVM_CAP_IOEVENTFD:
-       case KVM_CAP_DEVICE_CTRL:
        case KVM_CAP_USER_MEMORY:
        case KVM_CAP_SYNC_MMU:
        case KVM_CAP_DESTROY_MEMORY_REGION_WORKS:
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index e4890e04b210..191d220b6a30 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -567,7 +567,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
        case KVM_CAP_ENABLE_CAP:
        case KVM_CAP_S390_CSS_SUPPORT:
        case KVM_CAP_IOEVENTFD:
-       case KVM_CAP_DEVICE_CTRL:
        case KVM_CAP_S390_IRQCHIP:
        case KVM_CAP_VM_ATTRIBUTES:
        case KVM_CAP_MP_STATE:
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 13e88297f999..99e3da9ce42d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4525,6 +4525,15 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
        case KVM_CAP_BINARY_STATS_FD:
        case KVM_CAP_SYSTEM_EVENT_DATA:
                return 1;
+       case KVM_CAP_DEVICE_CTRL: {
+               int i;
+
+               for (i = 0; i < ARRAY_SIZE(kvm_device_ops_table); ++) {
+                       if (kvm_device_ops_table[i])
+                               return 1;
+               }
+               return 0;
+       }
        default:
                break;
        }