Re: Re: [PATCH v7 4/4] RISC-V: KVM: add KVM_CAP_RISCV_SET_HGATP_MODE

[fangyu . yu]

>On Thu, Apr 2, 2026 at 6:53 PM <fangyu.yu@xxxxxxxxxxxxxxxxx> wrote:
>>
>> From: Fangyu Yu <fangyu.yu@xxxxxxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxxxxxx>
>> Reviewed-by: Andrew Jones <andrew.jones@xxxxxxxxxxxxxxxx>
>> Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>
>> ---
>>  Documentation/virt/kvm/api.rst | 27 +++++++++++++++++++++++++++
>>  arch/riscv/kvm/vm.c            | 18 ++++++++++++++++--
>>  include/uapi/linux/kvm.h       |  1 +
>>  3 files changed, 44 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 032516783e96..9d7f6958fa81 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -8902,6 +8902,33 @@ 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
>> +: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.
>> +
>> +If this capability is not enabled, KVM will select the default HGATP mode
>> +automatically. The default is the highest HGATP.MODE value supported by
>> +hardware.
>> +
>> +``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. Supported-mode bitmask use HGATP.MODE
>> +encodings as defined by the RISC-V privileged specification, such as Sv39x4
>> +corresponds to HGATP.MODE=8, so userspace should test bitmask & BIT(8).
>> +
>>  8. Other capabilities.
>>  ======================
>>
>> diff --git a/arch/riscv/kvm/vm.c b/arch/riscv/kvm/vm.c
>> index 4d82a886102c..5e82a3ad3ad0 100644
>> --- a/arch/riscv/kvm/vm.c
>> +++ b/arch/riscv/kvm/vm.c
>> @@ -201,6 +201,9 @@ 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.pgd_levels);
>>                 break;
>> +       case KVM_CAP_RISCV_SET_HGATP_MODE:
>> +               r = kvm_riscv_get_hgatp_mode_mask();
>> +               break;
>
>Introducing a new RISC-V capability looks a bit complex.
>Instead of KVM_CAP_RISCV_SET_HGATP_MODE, we can
>simply re-use KVM_CAP_VM_GPA_BITS.
>
>The kvm_vm_ioctl_check_extension() for KVM_CAP_VM_GPA_BITS
>return number of GPA bits which in-directly implies the underlying
>hgatp.MODE. As we know, if it return 59 bits GPA then it means
>Sv57x4 is the selected hgatp.MODE and Sv48x4 and Sv39x4 modes
>are also supported as-per RISC-V privileged specification.
>
>The kvm_vm_ioctl_enable_cap() for KVM_CAP_VM_GPA_BITS
>will take the desired number of GPA bits and downsize the selected
>hgatp.MODE. For example, if user-space ask GPA bits <= 50 and
>GPA bits > 41 then we select Sv48x4. If user-space ask GPA
>bits <= 41 then we select Sv39x4. If user-space ask GPA bits <= 59
>and GPA bits > 50 then we select Sv57x4.
>

Thanks, that makes sense.

In v8 I’ll drop KVM_CAP_RISCV_SET_HGATP_MODE and re-use KVM_CAP_VM_GPA_BITS
for both discovery and selection.

Thanks,
Fangyu

>>         default:
>>                 r = 0;
>>                 break;
>> @@ -211,12 +214,23 @@ 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:
>> +               if (!kvm_riscv_hgatp_mode_is_valid(cap->args[0]))
>> +                       return -EINVAL;
>> +
>> +               if (kvm->created_vcpus || !kvm_are_all_memslots_empty(kvm))
>> +                       return -EBUSY;
>> +#ifdef CONFIG_64BIT
>> +               kvm->arch.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 80364d4dbebb..a74a80fd4046 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -989,6 +989,7 @@ struct kvm_enable_cap {
>>  #define KVM_CAP_ARM_SEA_TO_USER 245
>>  #define KVM_CAP_S390_USER_OPEREXEC 246
>>  #define KVM_CAP_S390_KEYOP 247
>> +#define KVM_CAP_RISCV_SET_HGATP_MODE 248
>>
>>  struct kvm_irq_routing_irqchip {
>>         __u32 irqchip;
>> --
>> 2.50.1
>>
>
>Regards,
>Anup