Re: [PATCH v10 5/5] drm/xe/xe_vm: Implement xe_vm_get_property_ioctl

[Raag Jadav <raag.jadav@intel.com>]

On Tue, Mar 25, 2025 at 03:01:27AM +0530, Cavitt, Jonathan wrote:
> From: Jadav, Raag <raag.jadav@intel.com> 
> > On Mon, Mar 24, 2025 at 10:27:08PM +0530, Cavitt, Jonathan wrote:
> > > From: Jadav, Raag <raag.jadav@intel.com> 
> > > > On Thu, Mar 20, 2025 at 03:26:15PM +0000, Jonathan Cavitt wrote:
> > > > > Add support for userspace to request a list of observed faults
> > > > > from a specified VM.
> > > > 
> > > > ...
> > > > 
> > > > > +static int xe_vm_get_property_size(struct xe_vm *vm, u32 property)
> > > > > +{
> > > > > +	int size = -EINVAL;
> > > > 
> > > > Mixing size and error codes is usually received with mixed feelings.
> > > > 
> > > > > +
> > > > > +	switch (property) {
> > > > > +	case DRM_XE_VM_GET_PROPERTY_FAULTS:
> > > > > +		spin_lock(&vm->faults.lock);
> > > > > +		size = vm->faults.len * sizeof(struct xe_vm_fault);
> > > > 
> > > > size_mul() and,
> > > > [1] perhaps fill it up into the pointer passed by the caller here?
> > > 
> > > "The pointer passed by the caller".  You mean the args pointer?
> > > 
> > > We'd still need to check that the args->size value is empty here before overwriting
> > > it, and we'd also still need to return the size to the ioctl so we can verify it's
> > > acceptable later in xe_vm_get_property_verify_size.
> > > 
> > > Unless you want to merge those two processes together into here?
> > 
> > The semantics are a bit fuzzy to me. Why do we have a single ioctl for
> > two different processes? Shouldn't they be handled separately?
> 
> No.  Sorry.  Let me clarify.
> "two different processes" = getting the size + verifying the size.

Yes, which seems like they should be handlded with _FAULT_NUM and
_FAULT_DATA ioctls but I guess we're way past it now.

I'm also not much informed about the history here. Is there a real
usecase behind exposing them? What is the user expected to do with
this information?

Raag