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

[Oliver Upton <oliver.upton@linux.dev>]

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 ;-)

-- 
Thanks,
Oliver