[PATCH v4 3/4] RISC-V: KVM: add KVM_CAP_RISCV_SET_HGATP_MODE

[Andrew Jones]

On Mon, Feb 02, 2026 at 10:07:15PM +0800, fangyu.yu at linux.alibaba.com wrote:
> From: Fangyu Yu <fangyu.yu at linux.alibaba.com>
> 
> Add a VM capability that allows userspace to select the G-stage page table
> format by setting HGATP.MODE on a per-VM basis.
> 
> Userspace enables the capability via KVM_ENABLE_CAP, passing the requested
> HGATP.MODE in args[0]. The request is rejected with -EINVAL if the mode is
> not supported by the host, and with -EBUSY if the VM has already been
> committed (e.g. vCPUs have been created or any memslot is populated).
> 
> KVM_CHECK_EXTENSION(KVM_CAP_RISCV_SET_HGATP_MODE) returns a bitmask of the
> HGATP.MODE formats supported by the host.
> 
> Signed-off-by: Fangyu Yu <fangyu.yu at linux.alibaba.com>
> ---
>  Documentation/virt/kvm/api.rst | 27 +++++++++++++++++++++++++++
>  arch/riscv/kvm/vm.c            | 20 ++++++++++++++++++--
>  include/uapi/linux/kvm.h       |  1 +
>  3 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 01a3abef8abb..1a0c5ddacae8 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -8765,6 +8765,33 @@ helpful if user space wants to emulate instructions which are not
>  This capability can be enabled dynamically even if VCPUs were already
>  created and are running.
>  
> +7.47 KVM_CAP_RISCV_SET_HGATP_MODE
> +---------------------------------
> +
> +:Architectures: riscv

If we only want this to work for rv64, then we should write riscv64 here,
but, as I said in the last patch, I think we can just support rv32 too
by supporting its one and only mode.

> +:Type: VM
> +:Parameters: args[0] contains the requested HGATP mode
> +:Returns:
> +  - 0 on success.
> +  - -EINVAL if args[0] is outside the range of HGATP modes supported by the
> +    hardware.
> +  - -EBUSY if vCPUs have already been created for the VM, if the VM has any
> +    non-empty memslots.
> +
> +This capability allows userspace to explicitly select the HGATP mode for
> +the VM. The selected mode must be supported by both KVM and hardware. This
> +capability must be enabled before creating any vCPUs or memslots.

We should write what happens if the capability (setting the mode) is not
done, i.e. what's the default mode.

> +
> +``KVM_CHECK_EXTENSION(KVM_CAP_RISCV_SET_HGATP_MODE)`` returns a bitmask of
> +HGATP.MODE values supported by the host. A return value of 0 indicates that
> +the capability is not supported.
> +
> +The returned bitmask uses the following bit positions::
> +
> +  bit 0: HGATP.MODE = SV39X4
> +  bit 1: HGATP.MODE = SV48X4
> +  bit 2: HGATP.MODE = SV57X4

Could write something along the lines of the UAPI having the bit
definitions rather than duplicating that information here.

> +
>  8. Other capabilities.
>  ======================
>  
> diff --git a/arch/riscv/kvm/vm.c b/arch/riscv/kvm/vm.c
> index 4b2156df40fc..3bbbcb6a17a6 100644
> --- a/arch/riscv/kvm/vm.c
> +++ b/arch/riscv/kvm/vm.c
> @@ -202,6 +202,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_VM_GPA_BITS:
>  		r = kvm_riscv_gstage_gpa_bits(&kvm->arch);
>  		break;
> +	case KVM_CAP_RISCV_SET_HGATP_MODE:
> +		r = kvm_riscv_get_hgatp_mode_mask();
> +		break;
>  	default:
>  		r = 0;
>  		break;
> @@ -212,12 +215,25 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  
>  int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
>  {
> +	if (cap->flags)
> +		return -EINVAL;
> +
>  	switch (cap->cap) {
>  	case KVM_CAP_RISCV_MP_STATE_RESET:
> -		if (cap->flags)
> -			return -EINVAL;
>  		kvm->arch.mp_state_reset = true;
>  		return 0;
> +	case KVM_CAP_RISCV_SET_HGATP_MODE:
> +#ifdef CONFIG_64BIT
> +		if (!kvm_riscv_hgatp_mode_is_valid(cap->args[0]))
> +			return -EINVAL;
> +
> +		if (kvm->created_vcpus || !kvm_are_all_memslots_empty(kvm))
> +			return -EBUSY;
> +
> +		kvm->arch.kvm_riscv_gstage_pgd_levels =
> +				3 + cap->args[0] - HGATP_MODE_SV39X4;
> +#endif
> +		return 0;
>  	default:
>  		return -EINVAL;
>  	}
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index dddb781b0507..00c02a880518 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -974,6 +974,7 @@ struct kvm_enable_cap {
>  #define KVM_CAP_GUEST_MEMFD_FLAGS 244
>  #define KVM_CAP_ARM_SEA_TO_USER 245
>  #define KVM_CAP_S390_USER_OPEREXEC 246
> +#define KVM_CAP_RISCV_SET_HGATP_MODE 247
>  
>  struct kvm_irq_routing_irqchip {
>  	__u32 irqchip;
> -- 
> 2.50.1
>

Thanks,
drew