vulkan: optimize UMA buffer operations and fix driver hangs (#16059)

* vulkan: optimize UMA buffer operations and fix driver hangs

The previous implementation was blocking the GPU for extended periods,
causing the i915 driver to reset the context due to the hangcheck
protection.

[32628.443070] i915 0000:00:02.0: [drm] GPU HANG: ecode 12:1:85dffffb, in llama-server [194114]
[32628.443091] i915 0000:00:02.0: [drm] llama-server[194114] context reset due to GPU hang

* vulkan: implement deferred_memset on UMA

---------

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

ggml/src/ggml-vulkan/ggml-vulkan.cpp

@@ -1185,6 +1185,14 @@ struct vk_staging_memcpy {
     size_t n;
 };
 
+struct vk_staging_memset {
+    vk_staging_memset(void * _dst, uint32_t _val, size_t _n) : dst(_dst), val(_val), n(_n) {}
+
+    void * dst;
+    uint32_t val;
+    size_t n;
+};
+
 struct vk_context_struct {
     vk_submission * s;
     std::vector<vk_sequence> seqs;
@@ -1193,6 +1201,7 @@ struct vk_context_struct {
 
     std::vector<vk_staging_memcpy> in_memcpys;
     std::vector<vk_staging_memcpy> out_memcpys;
+    std::vector<vk_staging_memset> memsets;
 
     vk_command_pool * p {};
 };
@@ -5196,6 +5205,14 @@ static void deferred_memcpy(void * dst, const void * src, size_t size, std::vect
     }
 }
 
+static void deferred_memset(void * dst, uint32_t val, size_t size, std::vector<vk_staging_memset>* memsets = nullptr) {
+    if (memsets == nullptr) {
+        memset(dst, val, size);
+    } else {
+        memsets->emplace_back(dst, val, size);
+    }
+}
+
 static void ggml_vk_ensure_sync_staging_buffer(vk_device& device, size_t size) {
     if (device->sync_staging == nullptr || device->sync_staging->size < size) {
         VK_LOG_MEMORY("ggml_vk_ensure_sync_staging_buffer(" << size << ")");
@@ -5391,6 +5408,10 @@ static void ggml_vk_buffer_write_2d(vk_buffer& dst, size_t offset, const void *
             memcpy(cpy.dst, cpy.src, cpy.n);
         }
 
+        for (auto& mset : subctx->memsets) {
+            memset(mset.dst, mset.val, mset.n);
+        }
+
         ggml_vk_submit(subctx, dst->device->fence);
         VK_CHECK(dst->device->device.waitForFences({ dst->device->fence }, true, UINT64_MAX), "vk_buffer_write_2d waitForFences");
         dst->device->device.resetFences({ dst->device->fence });
@@ -5530,12 +5551,25 @@ static void ggml_vk_buffer_copy(vk_buffer& dst, size_t dst_offset, vk_buffer& sr
 static void ggml_vk_buffer_memset_async(vk_context& ctx, vk_buffer& dst, size_t offset, uint32_t c, size_t size) {
     VK_LOG_DEBUG("ggml_vk_buffer_memset_async(" << offset << ", " << c << ", " << size << ")");
 
+    if (dst->memory_property_flags & vk::MemoryPropertyFlagBits::eHostVisible &&
+        dst->device->uma) {
+        deferred_memset((uint8_t*)dst->ptr + offset, c, size, &ctx->memsets);
+        return;
+    }
+
+    // Fall back to GPU fillBuffer for non-UMA or non-host-visible buffers
     ctx->s->buffer.fillBuffer(dst->buffer, offset, size, c);
 }
 
 static void ggml_vk_buffer_memset(vk_buffer& dst, size_t offset, uint32_t c, size_t size) {
     VK_LOG_DEBUG("ggml_vk_buffer_memset(" << offset << ", " << c << ", " << size << ")");
 
+    if (dst->memory_property_flags & vk::MemoryPropertyFlagBits::eHostVisible &&
+        dst->device->uma) {
+        memset((uint8_t*)dst->ptr + offset, c, size);
+        return;
+    }
+
     std::lock_guard<std::recursive_mutex> guard(dst->device->mutex);
     vk_context subctx = ggml_vk_create_temporary_context(dst->device->transfer_queue.cmd_pool);
     ggml_vk_ctx_begin(dst->device, subctx);
@@ -11170,6 +11204,10 @@ static bool ggml_vk_compute_forward(ggml_backend_vk_context * ctx, ggml_cgraph *
             memcpy(cpy.dst, cpy.src, cpy.n);
         }
 
+        for (auto& mset : subctx->memsets) {
+            memset(mset.dst, mset.val, mset.n);
+        }
+
         if (almost_ready && !ctx->almost_ready_fence_pending && !use_fence) {
             ggml_vk_submit(subctx, ctx->almost_ready_fence);
             ctx->almost_ready_fence_pending = true;
@@ -11192,6 +11230,7 @@ static bool ggml_vk_compute_forward(ggml_backend_vk_context * ctx, ggml_cgraph *
         }
         subctx->in_memcpys.clear();
         subctx->out_memcpys.clear();
+        subctx->memsets.clear();
     }
 
     return true;