Re: [PATCH v8 3/3] RISC-V: KVM: Reuse KVM_CAP_VM_GPA_BITS to select HGATP.MODE

[Nutty.Liu]

On 4/3/2026 11:30 PM, fangyu.yu@xxxxxxxxxxxxxxxxx wrote:
> From: Fangyu Yu <fangyu.yu@xxxxxxxxxxxxxxxxx>
>
> Reuse KVM_CAP_VM_GPA_BITS to advertise and select the effective
> G-stage GPA width for a VM.
>
> KVM_CHECK_EXTENSION(KVM_CAP_VM_GPA_BITS) returns the effective GPA
> bits for a VM, KVM_ENABLE_CAP(KVM_CAP_VM_GPA_BITS) allows userspace
> to downsize the effective GPA width by selecting a smaller G-stage
> page table format:
>    - gpa_bits <= 41 selects Sv39x4 (pgd_levels=3)
>    - gpa_bits <= 50 selects Sv48x4 (pgd_levels=4)
>    - gpa_bits <= 59 selects Sv57x4 (pgd_levels=5)
>
> Reject the request with -EINVAL for unsupported values and with -EBUSY
> if vCPUs have been created or any memslot is populated.
>
> Signed-off-by: Fangyu Yu <fangyu.yu@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Andrew Jones <andrew.jones@xxxxxxxxxxxxxxxx>
> Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>
Reviewed-by: Nutty Liu <nutty.liu@xxxxxxxxxxx>

Thanks,
Nutty
> ---
>   arch/riscv/kvm/vm.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 42 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/kvm/vm.c b/arch/riscv/kvm/vm.c
> index fb7c4e07961f..a9f083feeb76 100644
> --- a/arch/riscv/kvm/vm.c
> +++ b/arch/riscv/kvm/vm.c
> @@ -214,12 +214,52 @@ 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_VM_GPA_BITS: {
> +		unsigned long gpa_bits = cap->args[0];
> +		unsigned long new_levels;
> +		int r = 0;
> +
> +		/* Decide target pgd levels from requested gpa_bits */
> +#ifdef CONFIG_64BIT
> +		if (gpa_bits <= 41)
> +			new_levels = 3;        /* Sv39x4 */
> +		else if (gpa_bits <= 50)
> +			new_levels = 4;        /* Sv48x4 */
> +		else if (gpa_bits <= 59)
> +			new_levels = 5;        /* Sv57x4 */
> +		else
> +			return -EINVAL;
> +#else
> +		/* 32-bit: only Sv32x4*/
> +		if (gpa_bits <= 34)
> +			new_levels = 2;
> +		else
> +			return -EINVAL;
> +#endif
> +		if (new_levels > kvm_riscv_gstage_max_pgd_levels)
> +			return -EINVAL;
> +
> +		/* Follow KVM's lock ordering: kvm->lock -> kvm->slots_lock. */
> +		mutex_lock(&kvm->lock);
> +		mutex_lock(&kvm->slots_lock);
> +
> +		if (kvm->created_vcpus || !kvm_are_all_memslots_empty(kvm))
> +			r = -EBUSY;
> +		else
> +			kvm->arch.pgd_levels = new_levels;
> +
> +		mutex_unlock(&kvm->slots_lock);
> +		mutex_unlock(&kvm->lock);
> +
> +		return r;
> +	}
>   	default:
>   		return -EINVAL;
>   	}