linux-kernel - Re: [PATCH v5 3/3] RISC-V: KVM: add KVM_CAP_RISCV_SET_HGATP

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Message-ID: <ocfqo4zpsg6yyaz6kd65jrvudtb35uerdsueazqdk6w7c5lvdf@wvwzhc57gxez>
Date: Wed, 4 Feb 2026 09:32:10 -0600
From: Andrew Jones <andrew.jones@....qualcomm.com>
To: fangyu.yu@...ux.alibaba.com
Cc: pbonzini@...hat.com, corbet@....net, anup@...infault.org,
        atish.patra@...ux.dev, pjw@...nel.org, palmer@...belt.com,
        aou@...s.berkeley.edu, alex@...ti.fr, guoren@...nel.org,
        radim.krcmar@....qualcomm.com, linux-doc@...r.kernel.org,
        kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/3] RISC-V: KVM: add KVM_CAP_RISCV_SET_HGATP_MODE

On Wed, Feb 04, 2026 at 09:45:07PM +0800, fangyu.yu@...ux.alibaba.com wrote:
> From: Fangyu Yu <fangyu.yu@...ux.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@...ux.alibaba.com>
> ---
>  Documentation/virt/kvm/api.rst | 27 +++++++++++++++++++++++++++
>  arch/riscv/kvm/vm.c            | 19 +++++++++++++++++--
>  include/uapi/linux/kvm.h       |  1 +
>  3 files changed, 45 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 01a3abef8abb..62dc120857c1 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
> +: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.
> +

Currently the documentation for KVM_SET_ONE_REG has this for EBUSY

  EBUSY    (riscv) changing register value not allowed after the vcpu
           has run at least once

I suggest we update the KVM_SET_ONE_REG EBUSY description to say

(riscv) changing register value not allowed. This may occur after the vcpu
has run at least once or when other setup has completed which depends on
the value of the register.

> +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 4b2156df40fc..7d1e1d257df5 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 +215,24 @@ 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.kvm_riscv_gstage_pgd_levels =
> +				3 + cap->args[0] - HGATP_MODE_SV39X4;
> +#endif

 'if (IS_ENABLED(CONFIG_64BIT))' is preferred to the #ifdef.

> +		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