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

[Wang, Wei W]

On Saturday, December 17, 2022 1:13 AM, Sean Christopherson wrote:
> 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;
>         }

Yes, it looks better to move it to the generic check, but I'm not sure if it would be necessary to do the per-device check here either via CONFIG_KVM_VFIO (for example, if more non-arch-specific usages are added, we would end up with lots of such #ifdef to be added, which doesn't seem nice) or kvm_device_ops_table.

I think fundamentally KVM_CAP_DEVICE_CTRL is used to check if the generic kvm_device framework (e.g. KVM_CREATE_DEVICE) is supported by KVM (older KVM before 2013 doesn't have it). The per-device type (KVM_DEV_TYPE_VFIO, KVM_DEV_TYPE_ARM_PV_TIME etc.) support can be checked via KVM_CREATE_DEVICE, which reports -ENODEV if the device type doesn't have an entry in kvm_device_ops_table.