Re: [PATCH] drm/xe: Restrict user fences to long running VMs

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Jun 03, 2024 at 10:42:19PM +0200, Thomas Hellström wrote:
> Hi,
> 
> On Mon, 2024-06-03 at 10:53 -0700, Matthew Brost wrote:
> > User fences are intended to be used on long running VMs, enforce this
> > restriction. This addresses possible concerns of using user fences in
> > dma-fence and having the dma-fence signal before the user fence.
> 
> As mentioned in a separate thread, We should not introduce an uAPI
> change with the above motivation. We need to discuss potential use-

Sure. Agree this is really an orthogonal change to the other thread.
Just noticed we didn't have restriction when typing in that thread.

> cases for !LR vms and if there are found to be none, we could consider
> restricting in this way.

IMO we should strongly consider this as conceptually gives a clear
seperation of synchronization between non-LR and LR VMs.

Matt

> 
> /Thomas
> 
> 
> > 
> > Fixes: d1df9bfbf68c ("drm/xe: Only allow 1 ufence per exec / bind
> > IOCTL")
> > Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel
> > GPUs")
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >  drivers/gpu/drm/xe/xe_exec.c | 3 ++-
> >  drivers/gpu/drm/xe/xe_vm.c   | 3 ++-
> >  2 files changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/xe/xe_exec.c
> > b/drivers/gpu/drm/xe/xe_exec.c
> > index 97eeb973e897..a145813ad229 100644
> > --- a/drivers/gpu/drm/xe/xe_exec.c
> > +++ b/drivers/gpu/drm/xe/xe_exec.c
> > @@ -168,7 +168,8 @@ int xe_exec_ioctl(struct drm_device *dev, void
> > *data, struct drm_file *file)
> >  			num_ufence++;
> >  	}
> >  
> > -	if (XE_IOCTL_DBG(xe, num_ufence > 1)) {
> > +	if (XE_IOCTL_DBG(xe, num_ufence > 1) ||
> > +	    XE_IOCTL_DBG(xe, num_ufence && !xe_vm_in_lr_mode(vm))) {
> >  		err = -EINVAL;
> >  		goto err_syncs;
> >  	}
> > diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> > index 26b409e1b0f0..85da3a8a83b6 100644
> > --- a/drivers/gpu/drm/xe/xe_vm.c
> > +++ b/drivers/gpu/drm/xe/xe_vm.c
> > @@ -3226,7 +3226,8 @@ int xe_vm_bind_ioctl(struct drm_device *dev,
> > void *data, struct drm_file *file)
> >  			num_ufence++;
> >  	}
> >  
> > -	if (XE_IOCTL_DBG(xe, num_ufence > 1)) {
> > +	if (XE_IOCTL_DBG(xe, num_ufence > 1) ||
> > +	    XE_IOCTL_DBG(xe, num_ufence && !xe_vm_in_lr_mode(vm))) {
> >  		err = -EINVAL;
> >  		goto free_syncs;
> >  	}
>