From c3abe035c8171fbe0548286b1c371d940da83af6 Mon Sep 17 00:00:00 2001
From: "Justin C. Miller" <justin@justin.cm>
Date: Sun, 19 Jul 2020 17:01:15 -0700
Subject: [PATCH] [kernel] Remove thread_data pointer from TCB

The TCB is always stored at a constant offset within the thread object.
So instead of carrying an extra pointer, just implement thread::from_tcb
to get the thread.
---
 src/kernel/objects/thread.cpp   | 9 ++++++++-
 src/kernel/objects/thread.h     | 5 +++--
 src/kernel/scheduler.cpp        | 8 ++++----
 src/kernel/syscalls/object.cpp  | 2 +-
 src/kernel/syscalls/process.cpp | 8 ++++----
 5 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/src/kernel/objects/thread.cpp b/src/kernel/objects/thread.cpp
index b492b8c..6b9e48f 100644
--- a/src/kernel/objects/thread.cpp
+++ b/src/kernel/objects/thread.cpp
@@ -16,10 +16,17 @@ thread::thread(process &parent, uint8_t pri) :
 	TCB *tcbp = tcb();
 	tcbp->pml4 = parent.pml4();
 	tcbp->priority = pri;
-	tcbp->thread_data = this;
 	set_state(state::ready);
 }
 
+thread *
+thread::from_tcb(TCB *tcb)
+{
+	static ptrdiff_t offset =
+		-1 * static_cast<ptrdiff_t>(offsetof(thread, m_tcb));
+	return reinterpret_cast<thread*>(kutil::offset_pointer(tcb, offset));
+}
+
 void
 thread::wait_on_signals(kobject *obj, j6_signal_t signals)
 {
diff --git a/src/kernel/objects/thread.h b/src/kernel/objects/thread.h
index 325dca2..495897a 100644
--- a/src/kernel/objects/thread.h
+++ b/src/kernel/objects/thread.h
@@ -27,8 +27,6 @@ struct TCB
 
 	uint32_t time_left;
 	uint64_t last_ran;
-
-	thread *thread_data;
 };
 
 using tcb_list = kutil::linked_list<TCB>;
@@ -47,6 +45,9 @@ public:
 		none     = 0x00
 	};
 
+	/// Get the pointer to the thread object containing this TCB
+	static thread * from_tcb(TCB *tcb);
+
 	/// Get the `ready` state of the thread.
 	/// \returns True if the thread is ready to execute.
 	inline bool ready() const { return has_state(state::ready); }
diff --git a/src/kernel/scheduler.cpp b/src/kernel/scheduler.cpp
index feeb82c..87e2960 100644
--- a/src/kernel/scheduler.cpp
+++ b/src/kernel/scheduler.cpp
@@ -116,7 +116,7 @@ load_process_image(const void *image_start, size_t bytes, TCB *tcb)
 	// process code and load it
 	page_manager *pager = page_manager::get();
 
-	thread *th = tcb->thread_data;
+	thread *th = thread::from_tcb(tcb);
 
 	log::debug(logs::loader, "Loading task! ELF: %016lx [%d]", image_start, bytes);
 
@@ -279,7 +279,7 @@ void scheduler::prune(uint64_t now)
 	// move them to the appropriate lists.
 	auto *tcb = m_blocked.front();
 	while (tcb) {
-		thread *th = tcb->thread_data;
+		thread *th = thread::from_tcb(tcb);
 		uint8_t priority = tcb->priority;
 
 		bool ready = th->has_state(thread::state::ready);
@@ -311,7 +311,7 @@ scheduler::check_promotions(uint64_t now)
 {
 	for (auto &pri_list : m_runlists) {
 		for (auto *tcb : pri_list) {
-			const thread *th = m_current->thread_data;
+			const thread *th = thread::from_tcb(m_current);
 			const bool constant = th->has_state(thread::state::constant);
 			if (constant)
 				continue;
@@ -345,7 +345,7 @@ scheduler::schedule()
 	uint8_t priority = m_current->priority;
 	uint32_t remaining = m_apic->stop_timer();
 	m_current->time_left = remaining;
-	thread *th = m_current->thread_data;
+	thread *th = thread::from_tcb(m_current);
 	const bool constant = th->has_state(thread::state::constant);
 
 	if (remaining == 0) {
diff --git a/src/kernel/syscalls/object.cpp b/src/kernel/syscalls/object.cpp
index d1572ab..73cc12d 100644
--- a/src/kernel/syscalls/object.cpp
+++ b/src/kernel/syscalls/object.cpp
@@ -11,7 +11,7 @@ object_noop()
 {
 	auto &s = scheduler::get();
 	TCB *tcb = s.current();
-	thread *th = tcb->thread_data;
+	thread *th = thread::from_tcb(tcb);
 	log::debug(logs::syscall, "Thread %llx called noop syscall.", th->koid());
 	return j6_status_ok;
 }
diff --git a/src/kernel/syscalls/process.cpp b/src/kernel/syscalls/process.cpp
index 694c8db..fbbf294 100644
--- a/src/kernel/syscalls/process.cpp
+++ b/src/kernel/syscalls/process.cpp
@@ -11,7 +11,7 @@ process_exit(int64_t status)
 {
 	auto &s = scheduler::get();
 	TCB *tcb = s.current();
-	thread *th = tcb->thread_data;
+	thread *th = thread::from_tcb(tcb);
 	log::debug(logs::syscall, "Thread %llx exiting with code %d", th->koid(), status);
 
 	th->exit(status);
@@ -67,7 +67,7 @@ process_log(const char *message)
 
 	auto &s = scheduler::get();
 	TCB *tcb = s.current();
-	thread *th = tcb->thread_data;
+	thread *th = thread::from_tcb(tcb);
 	log::info(logs::syscall, "Message[%llx]: %s", th->koid(), message);
 	return j6_status_ok;
 }
@@ -80,7 +80,7 @@ process_pause()
 {
 	auto &s = scheduler::get();
 	TCB *tcb = s.current();
-	thread *th = tcb->thread_data;
+	thread *th = thread::from_tcb(tcb);
 	th->wait_on_signals(th, -1ull);
 	s.schedule();
 	return j6_status_ok;
@@ -91,7 +91,7 @@ process_sleep(uint64_t til)
 {
 	auto &s = scheduler::get();
 	TCB *tcb = s.current();
-	thread *th = tcb->thread_data;
+	thread *th = thread::from_tcb(tcb);
 	log::debug(logs::syscall, "Thread %llx sleeping until %llu", th->koid(), til);
 
 	th->wait_on_time(til);