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

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Message-Id: <20260120142256.9968-1-fangyu.yu@linux.alibaba.com>
Date: Tue, 20 Jan 2026 22:22:56 +0800
From: fangyu.yu@...ux.alibaba.com
To: radim.krcmar@....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>
>> ---
>> diff --git a/arch/riscv/kvm/vm.c b/arch/riscv/kvm/vm.c
>> @@ -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;
>>  	switch (cap->cap) {
>> +	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;
>
>"must be enabled before creating any vCPUs" check is missing.

Agreed, I'll add the missing "must be enabled before creating any vCPUs" check by
rejecting the capability once kvm->created_vcpus is non-zero.

>
>> +		kvm->arch.gstage_mode_initialized = true;
>> +		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;
>
>Even before creating VCPUs, I don't see enough protections to make this
>work.
>
>Userspace can only provide a hint about the physical address space size
>before any other KVM code could have acted on the information.
>It would be a serious issue if some code would operate on hgatp as if it
>were X and others as Y.
>
>The simplest solution would be to ensure that the CAP_SET VM ioctl can
>only be executed before any other IOCTL, but a change in generic code to
>achieve it would be frowned upon...
>I would recommend looking at kvm_are_all_memslots_empty() first, as it's
>quite likely that it could be sufficient for the purposes of changing
>hgatp.

Using kvm_are_all_memslots_empty might be a good idea, and I will add a
check for this function in the v2.

>
>Thanks.

Thanks,
Fangyu