Re: [PATCH 3/3] drm/xe: Allow scratch page under fault mode for certain platform

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Tue, 2025-01-28 at 17:21 -0500, Oak Zeng wrote:
> Normally scratch page is not allowed when a vm is operate under page
> fault mode, i.e., in the existing codes,
> DRM_XE_VM_CREATE_FLAG_SCRATCH_PAGE
> and DRM_XE_VM_CREATE_FLAG_FAULT_MODE are mutual exclusive. The reason
> is fault mode relies on recoverable page to work, while scratch page
> can mute recoverable page fault.
> 
> On xe2 and xe3, HW prefetching can cause page fault interrupt. Due to
> page fault interrupt overhead (i.e., need Guc and KMD involved to fix
> the page fault), HW prefetching can be slowed by many orders of
> magnitude.

The problem on xe2 and xe3 as I understand it is not the overhead but
the fact that OOB prefetching causes pagefaults and we can't resolve
those.

> 
> Fix this problem by allowing scratch page under fault mode for xe2
> and xe3.
> With scratch page in place, HW prefetching could always hit scratch
> page
> instead of causing interrupt.
> 
> Signed-off-by: Oak Zeng <oak.zeng@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_vm.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> index 196d347c6ac0..3346f88f284a 100644
> --- a/drivers/gpu/drm/xe/xe_vm.c
> +++ b/drivers/gpu/drm/xe/xe_vm.c
> @@ -1732,6 +1732,11 @@ int xe_vm_create_ioctl(struct drm_device *dev,
> void *data,
>  	if (XE_IOCTL_DBG(xe, args->extensions))
>  		return -EINVAL;
>  
> +	if (XE_IOCTL_DBG(xe, args->flags &
> DRM_XE_VM_CREATE_FLAG_SCRATCH_PAGE &&
> +			 args->flags &
> DRM_XE_VM_CREATE_FLAG_FAULT_MODE &&
> +			 !(IS_LUNARLAKE(xe) || IS_BATTLEMAGE(xe) ||
> IS_PANTHERLAKE(xe))))
> +		return -EINVAL;
> +

Could we perhaps add a bit in the xe->info structure?

/Thomas


>  	if (XE_WA(xe_root_mmio_gt(xe), 14016763929))
>  		args->flags |= DRM_XE_VM_CREATE_FLAG_SCRATCH_PAGE;
>  
> @@ -1745,10 +1750,6 @@ int xe_vm_create_ioctl(struct drm_device *dev,
> void *data,
>  	if (XE_IOCTL_DBG(xe, args->flags &
> ~ALL_DRM_XE_VM_CREATE_FLAGS))
>  		return -EINVAL;
>  
> -	if (XE_IOCTL_DBG(xe, args->flags &
> DRM_XE_VM_CREATE_FLAG_SCRATCH_PAGE &&
> -			 args->flags &
> DRM_XE_VM_CREATE_FLAG_FAULT_MODE))
> -		return -EINVAL;
> -
>  	if (XE_IOCTL_DBG(xe, !(args->flags &
> DRM_XE_VM_CREATE_FLAG_LR_MODE) &&
>  			 args->flags &
> DRM_XE_VM_CREATE_FLAG_FAULT_MODE))
>  		return -EINVAL;