From dd10667c56b943c19436c6540946f9ad0f71d257 Mon Sep 17 00:00:00 2001
From: Kimplul <kimi.h.kuparinen@gmail.com>
Date: Thu, 8 Jun 2023 18:43:18 +0300
Subject: add fast userspace return to do_ipc

---
 arch/riscv64/kernel/entry.S | 21 +++++++++++++++++++++
 common/uapi/ipc.c           |  1 +
 docs/doxygen.conf           |  3 ++-
 include/arch/proc.h         | 10 ++++++++++
 4 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/arch/riscv64/kernel/entry.S b/arch/riscv64/kernel/entry.S
index 32a04f6..2f9a9e6 100644
--- a/arch/riscv64/kernel/entry.S
+++ b/arch/riscv64/kernel/entry.S
@@ -149,3 +149,24 @@ _load_context:
 
 	/* assume supervisor for now */
 	sret
+
+.global ret_userspace_fast
+ret_userspace_fast:
+	lr	sp, offsetof_regs(tp)
+	addi	sp, sp, -sizeof_registers
+
+	lr	s0, offsetof_exec(tp)
+	csrw	CSR_SEPC, s0
+
+	csrw CSR_SSCRATCH, tp
+
+	lr	a0, offsetof_a0(sp)
+	lr      a1, offsetof_a1(sp)
+	lr      a2, offsetof_a2(sp)
+	lr      a3, offsetof_a3(sp)
+	lr      a4, offsetof_a4(sp)
+	lr      a5, offsetof_a5(sp)
+
+	lr	sp, offsetof_sp(sp)
+
+	sret
diff --git a/common/uapi/ipc.c b/common/uapi/ipc.c
index 36f2941..1d48f71 100644
--- a/common/uapi/ipc.c
+++ b/common/uapi/ipc.c
@@ -255,6 +255,7 @@ static void do_ipc(struct tcb *t,
 		t->eid = t->pid;
 
 	finalize_rpc(t, r, sd);
+	ret_userspace_fast();
 }
 /**
  * IPC request syscall handler.
diff --git a/docs/doxygen.conf b/docs/doxygen.conf
index 71a053c..3cb6ca3 100644
--- a/docs/doxygen.conf
+++ b/docs/doxygen.conf
@@ -2337,7 +2337,8 @@ INCLUDE_FILE_PATTERNS  =
 PREDEFINED             = DEBUG \
                          __GNUC__ \
                          riscv64 \
-                         riscv32
+                         riscv32 \
+			 ASSERT
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The
diff --git a/include/arch/proc.h b/include/arch/proc.h
index 493b168..c9b54db 100644
--- a/include/arch/proc.h
+++ b/include/arch/proc.h
@@ -113,4 +113,14 @@ void adjust_syscall(struct tcb *t);
  */
 __noreturn void run_init(struct tcb *t, void *fdt);
 
+/**
+ * Return to userspace fast.
+ * Doesn't load registers besides arguments.
+ * Mainly for do_ipc().
+ * Note that this skips canary checking,
+ * but at least do_ipc() uses basically zero stack space
+ * so I'm not too worried.
+ */
+__noreturn void ret_userspace_fast();
+
 #endif /* KMI_ARCH_PROC_H */
-- 
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