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

[fangyu.yu@linux.alibaba.com]

>> From: Fangyu Yu <fangyu.yu@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@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.

Agreed.

I'll update the documentation to list ":Architectures: riscv" and make the
capability available on both RV32 and RV64. For RV32, userspace will be
able to select the single supported G-stage mode (Sv32x4)

>> +: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.

Good point.
I'll update the documentation to explicitly state the default behavior when
KVM_CAP_RISCV_SET_HGATP_MODE is not used: KVM will auto-select the HGATP
G-stage mode during VM initialization. In other words, userspace only needs
to set the capability when it wants to override the default auto-selection.

>> +
>> +``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.
>

Sure. I'll rework this section.

>> +
>>  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

Thanks,
Fangyu