From 6fd403729d0e591eae99e27d9891692f318eaba8 Mon Sep 17 00:00:00 2001
From: KevinKickass <k@ppload.eu>
Date: Sat, 28 Feb 2026 17:48:37 +0100
Subject: [PATCH] [MOS] Fix SIGBUS crash on xe small BAR systems during
 vaInitialize

On systems where the CPU visible portion of VRAM is smaller than the
total VRAM size (small BAR), GEM objects allocated in VRAM may be
placed in non-CPU-accessible regions. When the driver later attempts
to mmap and access these buffers, this results in a SIGBUS crash.

This affects discrete GPUs like DG2 (Arc A750/A770) on platforms
without Resizable BAR support, where only 256MB of 8GB VRAM is
CPU accessible.

Fix by detecting small BAR during buffer manager initialization by
comparing cpu_visible_size against total_size for VRAM regions.
When small BAR is detected, set DRM_XE_GEM_CREATE_FLAG_NEEDS_VISIBLE_VRAM
on VRAM allocations to ensure the kernel places them in CPU visible
VRAM, and add system memory as a fallback placement so the kernel
can spill if CPU visible VRAM is full. On full BAR systems, this
flag is a noop per the xe KMD documentation.

Fixes: vaInitialize -> mos_bufmgr_gem_init_xe -> mos_bo_alloc_xe ->
       mmap SIGBUS on DG2 with xe driver and small BAR (256MB).
---
 .../linux/common/os/xe/mos_bufmgr_xe.c        | 48 +++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/media_softlet/linux/common/os/xe/mos_bufmgr_xe.c b/media_softlet/linux/common/os/xe/mos_bufmgr_xe.c
index 026d7b0d72..ce991b0b5a 100644
--- a/media_softlet/linux/common/os/xe/mos_bufmgr_xe.c
+++ b/media_softlet/linux/common/os/xe/mos_bufmgr_xe.c
@@ -233,6 +233,7 @@ typedef struct mos_xe_bufmgr_gem {
 
     //Note: DON't put these fields in xe_device
     bool     has_vram;
+    bool     has_small_bar;
     uint8_t  va_bits;
     /** bitmask of all memory regions */
     uint64_t mem_regions_mask;
@@ -722,6 +723,39 @@ bool __mos_has_vram_xe(struct mos_bufmgr *bufmgr)
     return has_vram;
 }
 
+static bool __mos_has_small_bar_xe(struct mos_bufmgr *bufmgr)
+{
+    MOS_DRM_CHK_NULL_RETURN_VALUE(bufmgr, false)
+    struct mos_xe_bufmgr_gem *bufmgr_gem = (struct mos_xe_bufmgr_gem *)bufmgr;
+    struct mos_xe_device *dev = &bufmgr_gem->xe_device;
+    int fd = bufmgr_gem->fd;
+
+    if (!bufmgr_gem->has_vram)
+    {
+        bufmgr_gem->has_small_bar = false;
+        return false;
+    }
+
+    MOS_DRM_CHK_XE_DEV(dev, mem_regions, __mos_query_mem_regions_xe, false)
+
+    struct drm_xe_query_mem_regions *mem_regions = dev->mem_regions;
+    for (int i = 0; i < mem_regions->num_mem_regions; i++)
+    {
+        if (mem_regions->mem_regions[i].mem_class == DRM_XE_MEM_REGION_CLASS_VRAM &&
+            mem_regions->mem_regions[i].cpu_visible_size < mem_regions->mem_regions[i].total_size)
+        {
+            bufmgr_gem->has_small_bar = true;
+            MOS_DRM_NORMALMESSAGE("Small BAR detected: VRAM total %llu, CPU visible %llu",
+                (unsigned long long)mem_regions->mem_regions[i].total_size,
+                (unsigned long long)mem_regions->mem_regions[i].cpu_visible_size);
+            return true;
+        }
+    }
+
+    bufmgr_gem->has_small_bar = false;
+    return false;
+}
+
 uint8_t __mos_query_va_bits_xe(struct mos_bufmgr *bufmgr)
 {
     uint8_t va_bits = 48;
@@ -1337,6 +1371,19 @@ mos_bo_alloc_xe(struct mos_bufmgr *bufmgr,
     {
         //Note: memory_region is related to gt_id for multi-tiles gpu, take gt_id into consideration in case of multi-tiles
         create.placement = bufmgr_gem->mem_regions_mask & (~0x1);
+
+        if (bufmgr_gem->has_small_bar)
+        {
+            /**
+             * On small BAR systems, only a portion of VRAM is CPU accessible.
+             * Set NEEDS_VISIBLE_VRAM to ensure the allocation lands in the
+             * CPU visible part of VRAM. Also add system memory as a fallback
+             * placement so the kernel can spill if CPU visible VRAM is full.
+             * On full BAR systems this flag is a noop.
+             */
+            create.flags |= DRM_XE_GEM_CREATE_FLAG_NEEDS_VISIBLE_VRAM;
+            create.placement |= bufmgr_gem->mem_regions_mask & 0x1;
+        }
     }
     else
     {
@@ -3677,6 +3724,7 @@ mos_bufmgr_gem_init_xe(int fd, int batch_size)
     bufmgr_gem->vm_id = __mos_vm_create_xe(&bufmgr_gem->bufmgr);
     __mos_query_mem_regions_instance_mask_xe(&bufmgr_gem->bufmgr);
     __mos_has_vram_xe(&bufmgr_gem->bufmgr);
+    __mos_has_small_bar_xe(&bufmgr_gem->bufmgr);
     __mos_get_default_alignment_xe(&bufmgr_gem->bufmgr);
 
     DRMLISTADD(&bufmgr_gem->managers, &bufmgr_list);