Re: [PATCH v10 0/3] Allow scratch page under fault mode for certain platform

["Ghimiray, Himal Prasad" <himal.prasad.ghimiray@intel.com>]

On 03-04-2025 22:23, 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, out of bound prefetch can cause page fault and further
> system hang because xekmd can't resolve such page fault. SYCL and OCL
> language runtime requires out of bound prefetch to be silently dropped
> without causing any functional problem, thus the existing behavior
> doesn't meet language runtime requirement.
> 
> At the same time, 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.
> 
> Fix those problems 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.
> 
> A side effect is, scratch page could hide application program error.
> Application out of bound accesses are hided by scratch page mapping,
> instead of get reported to user.
> 
> igt test: https://patchwork.freedesktop.org/series/144907/, Test result on
> BMG:

pushed to drm-xe-next.

Thanks for the patches and Reviews.>
> root@DUT1130BMGFRD:/home/szeng/dii-tools/igt-public/build/tests# ./xe_exec_fault_mode --run-subtest scratch-fault
> IGT-Version: 1.30-gde1a3cb42 (x86_64) (Linux: 6.13.0-xe x86_64)
> Using IGT_SRANDOM=1738684805 for randomisation
> Opened device: /dev/dri/card0
> Starting subtest: scratch-fault
> Subtest scratch-fault: SUCCESS (0.080s)
> 
> Oak Zeng (3):
>    drm/xe: Introduced needs_scratch bit in device descriptor
>    drm/xe: Clear scratch page on vm_bind
>    drm/xe: Allow scratch page under fault mode for certain platform
> 
>   drivers/gpu/drm/xe/xe_device_types.h |  2 +
>   drivers/gpu/drm/xe/xe_pci.c          |  5 ++
>   drivers/gpu/drm/xe/xe_pt.c           | 99 ++++++++++++++++++----------
>   drivers/gpu/drm/xe/xe_vm.c           | 31 +++++++--
>   drivers/gpu/drm/xe/xe_vm_types.h     |  2 +
>   include/uapi/drm/xe_drm.h            |  6 +-
>   6 files changed, 102 insertions(+), 43 deletions(-)
>