From 0a8ac9fe4d0d0cf86da8eaa4c9d995873dd191ef Mon Sep 17 00:00:00 2001
From: Sebastian Sumpf <sebastian.sumpf@genode-labs.com>
Date: Wed, 1 Feb 2023 17:27:46 +0100
Subject: [PATCH] gpu/intel: don't copy sesion cap for onwership check

We determine the owner of a Vram object by the session cap of the
session that created the object. We should not copy this cap to other
places because this increases reference counting, that can become very
large with many objects. Therefore, we pass a wrapper 'Owner' object
containing the cap by reference.

issue #4713
---
 repos/os/src/drivers/gpu/intel/main.cc | 89 ++++++++++++++------------
 1 file changed, 49 insertions(+), 40 deletions(-)

diff --git a/repos/os/src/drivers/gpu/intel/main.cc b/repos/os/src/drivers/gpu/intel/main.cc
index 0bbbe51ded..044b86b4b1 100644
--- a/repos/os/src/drivers/gpu/intel/main.cc
+++ b/repos/os/src/drivers/gpu/intel/main.cc
@@ -1571,12 +1571,56 @@ class Gpu::Session_component : public Genode::Session_object<Gpu::Session>
 {
 	private:
 
+		/*
+		 * Vram managed by session ep
+		 */
+		struct Vram : Rpc_object<Gpu::Vram>
+		{
+			/*
+			 * Do not copy session cap, since this increases the session-cap reference
+			 * count for each Vram object.
+			 */
+			struct Owner {
+				Capability<Gpu::Session> cap;
+			};
+
+			Ram_dataspace_capability ds_cap;
+			Owner const             &owning_session;
+
+			enum { INVALID_FENCE = 0xff };
+			Genode::uint32_t fenced { INVALID_FENCE };
+
+			Igd::Ggtt::Mapping map { };
+
+			addr_t phys_addr { 0 };
+			size_t size { 0 };
+
+			bool   caps_used { false };
+			size_t ram_used { 0 };
+
+			Vram(Ram_dataspace_capability ds_cap, Genode::addr_t phys_addr,
+			     Owner const &owner)
+			:
+				ds_cap { ds_cap }, owning_session { owner },
+				phys_addr { phys_addr }
+			{
+				Dataspace_client buf(ds_cap);
+				size = buf.size();
+			}
+
+			bool owner(Capability<Gpu::Session> const other)
+			{
+				return owning_session.cap == other;
+			}
+		};
+
 		Genode::Env              &_env;
 		Genode::Region_map       &_rm;
 		Constrained_ram_allocator _ram;
 		Igd::Device              &_device;
 		Heap                      _heap { _device._pci_backend_alloc, _rm };
-		Capability<Gpu::Session>  _session_cap { cap() };
+		/* used to mark ownership of an allocated VRAM object */
+		Vram::Owner               _owner { cap() };
 
 		Igd::Device::Vgpu  _vgpu;
 
@@ -1636,41 +1680,6 @@ class Gpu::Session_component : public Genode::Session_object<Gpu::Session>
 
 		Resource_guard _resource_guard { _cap_quota_guard(), _ram_quota_guard() };
 
-		/*
-		 * Vram managed by session ep
-		 */
-		struct Vram : Rpc_object<Gpu::Vram>
-		{
-			Ram_dataspace_capability ds_cap;
-			Session_capability       owner_cap;
-
-			enum { INVALID_FENCE = 0xff };
-			Genode::uint32_t fenced { INVALID_FENCE };
-
-			Igd::Ggtt::Mapping map { };
-
-			addr_t phys_addr { 0 };
-			size_t size { 0 };
-
-			bool   caps_used { false };
-			size_t ram_used { 0 };
-
-			Vram(Ram_dataspace_capability ds_cap, Genode::addr_t phys_addr,
-			     Session_capability  owner_cap)
-			:
-				ds_cap { ds_cap }, owner_cap { owner_cap },
-				phys_addr { phys_addr }
-			{
-				Dataspace_client buf(ds_cap);
-				size = buf.size();
-			}
-
-			bool owner(Capability<Gpu::Session> other)
-			{
-				return owner_cap == other;
-			}
-		};
-
 		/*
 		 * Vram session/gpu-context local vram
 		 */
@@ -1796,7 +1805,7 @@ class Gpu::Session_component : public Genode::Session_object<Gpu::Session>
 
 				_apply_vram(vram_local, [&](Vram &vram) {
 
-					if (vram.owner(_session_cap) == false) return false;
+					if (vram.owner(cap()) == false) return false;
 
 					if (vram.map.offset != Igd::Ggtt::Mapping::INVALID_OFFSET) {
 						_device.unmap_vram(_heap, vram.map);
@@ -1912,7 +1921,7 @@ class Gpu::Session_component : public Genode::Session_object<Gpu::Session>
 
 			Ram_dataspace_capability ds_cap = _device.alloc_vram(_heap, size);
 			addr_t phys_addr                = _device.dma_addr(ds_cap);
-			Vram *vram                  = new (&_heap) Vram(ds_cap, phys_addr, _session_cap);
+			Vram *vram                      = new (&_heap) Vram(ds_cap, phys_addr, _owner);
 			_env.ep().manage(*vram);
 
 			try {
@@ -1942,7 +1951,7 @@ class Gpu::Session_component : public Genode::Session_object<Gpu::Session>
 
 				_apply_vram(vram_local, [&](Vram &vram) {
 
-					if (vram.owner(_session_cap) == false) return false;
+					if (vram.owner(cap()) == false) return false;
 
 					if (vram.map.offset != Igd::Ggtt::Mapping::INVALID_OFFSET) {
 						Genode::error("cannot free mapped vram");
@@ -2088,7 +2097,7 @@ class Gpu::Session_component : public Genode::Session_object<Gpu::Session>
 
 			Vram *v = nullptr;
 			auto lookup = [&] (Vram &vram) {
-				if (!vram.map.cap.valid() || !vram.owner(_session_cap)) { return false; }
+				if (!vram.map.cap.valid() || !vram.owner(cap())) { return false; }
 				v = &vram;
 				return false;
 			};