[PATCH v3 i-g-t] lib/intel_pat: Cache PAT config for unprivileged processes

[himanshu.girotra@intel.com]

From: Himanshu Girotra <himanshu.girotra@intel.com>

Commit 4e59b8e7779b ("lib/intel_pat: use kernel debugfs as
authoritative PAT source for Xe") changed the Xe PAT lookup to
read from debugfs, which requires root access. Some xe_oa subtests
fork a child process that drops root privileges before performing
GPU operations that internally need the PAT write-back index.
After dropping root, the debugfs read fails.

Fix this by caching the PAT configuration in struct xe_device at
xe_device_get() time. Since xe_device_get() is called while still
root, the cache is available to forked children even after
igt_drop_root(). Tests that open a new fd after forking must call
xe_device_get() explicitly before dropping root to populate the
cache for that fd.

v2: Cache PAT in struct xe_device instead of a global static;
    drop the intel_pat_precache() helper — xe_device_get() already
    populates the cache, so call it directly in xe_oa before
    igt_drop_root() (Ashutosh Dixit)

v3: Use pat_cache pointer, NULL indicates unpopulated cache;
    remove redundant xe_oa.c change (Ashutosh Dixit)

    Add FIXME for multi-GPU (Zbigniew Kempczyński)

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Xin Wang <x.wang@intel.com>
Cc: Nishit Sharma <nishit.sharma@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>

Reviewed-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Signed-off-by: Himanshu Girotra <himanshu.girotra@intel.com>
---
 lib/intel_pat.c   | 18 ++++++++++--------
 lib/xe/xe_query.c | 20 ++++++++++++++++++++
 lib/xe/xe_query.h |  5 +++++
 3 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/lib/intel_pat.c b/lib/intel_pat.c
index 8660a2515..e758ce9be 100644
--- a/lib/intel_pat.c
+++ b/lib/intel_pat.c
@@ -6,6 +6,7 @@
 #include <fcntl.h>
 #include "igt.h"
 #include "intel_pat.h"
+#include "xe/xe_query.h"
 
 /**
  * xe_get_pat_sw_config - Helper to read PAT (Page Attribute Table) software configuration
@@ -99,17 +100,18 @@ static void intel_get_pat_idx(int fd, struct intel_pat_cache *pat)
 	uint16_t dev_id;
 
 	/*
-	 * For Xe driver, query the kernel's PAT software configuration
-	 * via debugfs. The kernel is the authoritative source for PAT
-	 * indices, accounting for platform-specific workarounds
-	 * (e.g. Wa_16023588340) at runtime.
+	 * For Xe, use the PAT cache stored in struct xe_device.
+	 * xe_device_get() populates the cache while still root; forked
+	 * children that inherit the xe_device can use it post-drop_root().
 	 */
 	if (is_xe_device(fd)) {
-		int32_t parsed = xe_get_pat_sw_config(fd, pat);
+		struct xe_device *xe_dev = xe_device_get(fd);
 
-		igt_assert_f(parsed > 0,
-			     "Failed to get PAT sw_config from debugfs (parsed=%d)\n",
-			     parsed);
+		igt_assert_f(xe_dev->pat_cache,
+			     "PAT sw_config not available for fd %d -- "
+			     "debugfs not accessible (missing root or not mounted?)\n",
+			     fd);
+		*pat = *xe_dev->pat_cache;
 		return;
 	}
 
diff --git a/lib/xe/xe_query.c b/lib/xe/xe_query.c
index 981d76948..ea8889f3b 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"
 
@@ -235,6 +236,7 @@ static void xe_device_free(struct xe_device *xe_dev)
 	free(xe_dev->mem_regions);
 	free(xe_dev->vram_size);
 	free(xe_dev->eu_stall);
+	free(xe_dev->pat_cache);
 	free(xe_dev);
 }
 
@@ -299,6 +301,24 @@ 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(). pat_cache is left NULL if
+	 * debugfs is not accessible.
+	 *
+	 * FIXME: the cache is keyed by fd; for multi-GPU support this
+	 * should be extended to cache PAT entries by platform version/
+	 * revision instead.
+	 */
+	xe_dev->pat_cache = calloc(1, sizeof(*xe_dev->pat_cache));
+	igt_assert(xe_dev->pat_cache);
+	if (xe_get_pat_sw_config(xe_dev->fd, xe_dev->pat_cache) <= 0) {
+		free(xe_dev->pat_cache);
+		xe_dev->pat_cache = NULL;
+	}
+
 	/* 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..53a32e4b1 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 +76,9 @@ struct xe_device {
 
 	/** @dev_id: Device id of xe device */
 	uint16_t dev_id;
+
+	/** @pat_cache: cached PAT index configuration, NULL if not yet populated */
+	struct intel_pat_cache *pat_cache;
 };
 
 #define xe_for_each_engine(__fd, __hwe) \
-- 
2.50.1