linux-kernel - Re: Re: [PATCH v2] RISC-V: KVM: add KVM_CAP_RISCV_SET_HGATP

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Message-Id: <20260116142947.30520-1-fangyu.yu@linux.alibaba.com>
Date: Fri, 16 Jan 2026 22:29:47 +0800
From: fangyu.yu@...ux.alibaba.com
To: andrew.jones@....qualcomm.com
Cc: ajones@...tanamicro.com,
	alex@...ti.fr,
	anup@...infault.org,
	aou@...s.berkeley.edu,
	atish.patra@...ux.dev,
	corbet@....net,
	fangyu.yu@...ux.alibaba.com,
	guoren@...nel.org,
	kvm-riscv@...ts.infradead.org,
	kvm@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org,
	palmer@...belt.com,
	pbonzini@...hat.com,
	pjw@...nel.org,
	rkrcmar@...tanamicro.com
Subject: Re: Re: [PATCH v2] RISC-V: KVM: add KVM_CAP_RISCV_SET_HGATP_MODE

>> From: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
>>
>> This capability allows userspace to explicitly select the HGATP mode
>> for the VM. The selected mode must be less than or equal to the max
>> HGATP mode supported by the hardware. This capability must be enabled
>> before creating any vCPUs, and can only be set once per VM.
>>
>> Signed-off-by: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
>> ---
>>  Documentation/virt/kvm/api.rst | 14 ++++++++++++++
>>  arch/riscv/kvm/vm.c            | 26 ++++++++++++++++++++++++--
>>  include/uapi/linux/kvm.h       |  1 +
>>  3 files changed, 39 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 01a3abef8abb..9e17788e3a9d 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -8765,6 +8765,20 @@ 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 arg[0] is outside the range of hgatp
>> +          modes supported by the hardware.
>> +
>> +This capability allows userspace to explicitly select the HGATP mode for
>> +the VM. The selected mode must be less than or equal to the maximum HGATP
>> +mode supported by the hardware. This capability must be enabled before
>> +creating any vCPUs, and can only be set once per VM.
>
>I think I would prefer a KVM_CAP_RISCV_SET_MAX_GPA type of capability. The
>reason is because, while one of the results of the max-gpa being set will
>be to set hgatp, there may be other reasons to track the guest's maximum
>physical address too and kvm userspace shouldn't need to think about each
>individually.

That makes sense, thanks.

>> +
>>  8. Other capabilities.
>>  ======================
>>
>> diff --git a/arch/riscv/kvm/vm.c b/arch/riscv/kvm/vm.c
>> index 4b2156df40fc..e9275023a73a 100644
>> --- a/arch/riscv/kvm/vm.c
>> +++ b/arch/riscv/kvm/vm.c
>> @@ -202,6 +202,13 @@ 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:
>> +#ifdef CONFIG_64BIT
>> +		r = 1;
>> +#else/* CONFIG_32BIT */
>> +		r = 0;
>> +#endif
>
> r = IS_ENABLED(CONFIG_64BIT) ? 1 : 0;

Ack.

>> +		break;
>>  	default:
>>  		r = 0;
>>  		break;
>> @@ -212,12 +219,27 @@ 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;
>
>add blank line

Ack, will add a blank line after the flags check.

>
>>  	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 (cap->args[0] < HGATP_MODE_SV39X4 ||
>> +			cap->args[0] > kvm_riscv_gstage_max_mode)
>> +			return -EINVAL;
>> +		if (kvm->arch.gstage_mode_initialized)
>> +			return 0;
>
>I think we want to return -EBUSY here and it should be documented where it
>already states "...can only be set once per VM"

Agreed.

>> +		kvm->arch.gstage_mode_initialized = true;
>
>In the previous patch I thought we were missing this, but I see now it
>means "user initialized". Let's rename it as such,
>
> gstage_mode_user_initialized

Agreed.

>> +		kvm->arch.kvm_riscv_gstage_mode = cap->args[0];
>> +		kvm->arch.kvm_riscv_gstage_pgd_levels = 3 +
>> +		    kvm->arch.kvm_riscv_gstage_mode - HGATP_MODE_SV39X4;
>> +		kvm_info("using SV%lluX4 G-stage page table format\n",
>> +			39 + (cap->args[0] - HGATP_MODE_SV39X4) * 9);
>> +#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