Re: [PATCH v2 i-g-t] lib/intel_pat: Cache PAT config for unprivileged processes

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Thu, 05 Mar 2026 04:25:48 -0800, <himanshu.girotra@intel.com> wrote:
>
> diff --git a/lib/xe/xe_query.c b/lib/xe/xe_query.c
> index 981d76948..256ec1d4d 100644
> --- a/lib/xe/xe_query.c
> +++ b/lib/xe/xe_query.c
> @@ -22,6 +22,7 @@
>  #include "ioctl_wrappers.h"
>  #include "igt_map.h"
>
> +#include "intel_pat.h"
>  #include "xe_query.h"
>  #include "xe_ioctl.h"
>
> @@ -299,6 +300,15 @@ struct xe_device *xe_device_get(int fd)
>	xe_dev->default_alignment = __mem_default_alignment(xe_dev->mem_regions);
>	xe_dev->has_vram = __mem_has_vram(xe_dev->mem_regions);
>
> +	/*
> +	 * Populate the PAT cache while we still have sufficient privileges
> +	 * to read debugfs.  Forked children that inherit this xe_device
> +	 * (via fork()) will be able to use the cached values even after
> +	 * dropping root with igt_drop_root().
> +	 */
> +	if (xe_get_pat_sw_config(xe_dev->fd, &xe_dev->pat_cache) > 0)
> +		xe_dev->pat_cached = true;
> +
x>	/* We may get here from multiple threads, use first cached xe_dev */
>	pthread_mutex_lock(&cache.cache_mutex);
>	prev = find_in_cache_unlocked(fd);
> diff --git a/lib/xe/xe_query.h b/lib/xe/xe_query.h
> index d7a9f95f9..97bcf0d32 100644
> --- a/lib/xe/xe_query.h
> +++ b/lib/xe/xe_query.h
> @@ -9,6 +9,8 @@
>  #ifndef XE_QUERY_H
>  #define XE_QUERY_H
>
> +#include "intel_pat.h"
> +
>  #include <stdint.h>
>  #include <xe_drm.h>
>
> @@ -74,6 +76,12 @@ struct xe_device {
>
>	/** @dev_id: Device id of xe device */
>	uint16_t dev_id;
> +
> +	/** @pat_cache: cached PAT index configuration */
> +	struct intel_pat_cache pat_cache;

This should just be "struct intel_pat_cache *pat_cache";

> +
> +	/** @pat_cached: true once @pat_cache has been populated */
> +	bool pat_cached;

And then we don't need this, we can just check for NULL pointer, like has
been done for other members of 'struct xe_device'.

>  };
>
>  #define xe_for_each_engine(__fd, __hwe) \
> diff --git a/tests/intel/xe_oa.c b/tests/intel/xe_oa.c
> index 927f3f4f2..9ce828619 100644
> --- a/tests/intel/xe_oa.c
> +++ b/tests/intel/xe_oa.c
> @@ -3453,6 +3453,14 @@ test_single_ctx_render_target_writes_a_counter(const struct drm_xe_oa_unit *oau)
>			/* A local device for local resources. */
>			drm_fd = drm_reopen_driver(drm_fd);
>
> +			/*
> +			 * Populate the xe_device cache (including PAT) for
> +			 * the new fd while we still have root privileges.
> +			 * intel_get_pat_idx_*() reads PAT from xe_device,
> +			 * so this must happen before igt_drop_root().
> +			 */
> +			xe_device_get(drm_fd);

This is not needed, this is done in igt_main() in xe_oa.c. Also you have
done a xe_device_get() in lib/intel_pat.c in this patch itself.

> +
>			igt_drop_root();
>
>			single_ctx_helper(oau);
> --
> 2.50.1
>