Re: [PATCH 03/11] KVM: arm64: Add vm fd device attribute accessors

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 21/03/2023 16:49, Oliver Upton wrote:
> Hi Suzuki,
> 
> On Tue, Mar 21, 2023 at 09:53:06AM +0000, Suzuki K Poulose wrote:
>> On 20/03/2023 22:09, Oliver Upton wrote:
>>> A subsequent change will allow userspace to convey a filter for
>>> hypercalls through a vm device attribute. Add the requisite boilerplate
>>> for vm attribute accessors.
>>>
>>> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
>>> ---
>>>    arch/arm64/kvm/arm.c | 29 +++++++++++++++++++++++++++++
>>>    1 file changed, 29 insertions(+)
>>>
>>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>>> index 3bd732eaf087..b6e26c0e65e5 100644
>>> --- a/arch/arm64/kvm/arm.c
>>> +++ b/arch/arm64/kvm/arm.c
>>> @@ -1439,11 +1439,28 @@ static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm,
>>>    	}
>>>    }
>>> +static int kvm_vm_has_attr(struct kvm *kvm, struct kvm_device_attr *attr)
>>> +{
>>> +	switch (attr->group) {
>>> +	default:
>>> +		return -ENXIO;
>>> +	}
>>> +}
>>> +
>>> +static int kvm_vm_set_attr(struct kvm *kvm, struct kvm_device_attr *attr)
>>> +{
>>> +	switch (attr->group) {
>>> +	default:
>>> +		return -ENXIO;
>>> +	}
>>> +}
>>> +
>>>    long kvm_arch_vm_ioctl(struct file *filp,
>>>    		       unsigned int ioctl, unsigned long arg)
>>>    {
>>>    	struct kvm *kvm = filp->private_data;
>>>    	void __user *argp = (void __user *)arg;
>>> +	struct kvm_device_attr attr;
>>>    	switch (ioctl) {
>>>    	case KVM_CREATE_IRQCHIP: {
>>> @@ -1479,6 +1496,18 @@ long kvm_arch_vm_ioctl(struct file *filp,
>>>    			return -EFAULT;
>>>    		return kvm_vm_ioctl_mte_copy_tags(kvm, &copy_tags);
>>>    	}
>>> +	case KVM_HAS_DEVICE_ATTR: {
>>> +		if (copy_from_user(&attr, argp, sizeof(attr)))
>>> +			return -EFAULT;
>>> +
>>> +		return kvm_vm_has_attr(kvm, &attr);
>>> +	}
>>> +	case KVM_SET_DEVICE_ATTR: {
>>> +		if (copy_from_user(&attr, argp, sizeof(attr)))
>>> +			return -EFAULT;
>>> +
>>> +		return kvm_vm_set_attr(kvm, &attr);
>>> +	}
>>
>> Is there a reason to exclude KVM_GET_DEVICE_ATTR handling ?
> 
> The GET_DEVICE_ATTR would effectively be dead code, as the hypercall filter is
> a write-only attribute. The filter is constructed through iterative calls to
> the attribute, so conveying the end result to userspace w/ the same UAPI
> is non-trivial.
> 
> Hopefully userspace remembers what it wrote to the field ;-)

Fair enough. I am thinking of using VM DEVICE_ATTR for some of the Arm
CCA Realm Configuration, which are now overloaded with ENABLE_CAP.

We could add GET_DEVICE_ATTR if and when we need it.

Thanks
Suzuki


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