From 9fc9f755e1f9ec56a452b334f6112028e0ec0329 Mon Sep 17 00:00:00 2001
From: Robert Morris <rtm@csail.mit.edu>
Date: Tue, 9 Aug 2022 11:44:02 -0400
Subject: adopt github PR98 (No need to store TRAPFRAME in sscratch register)

---
 kernel/trampoline.S | 37 +++++++++++++++++++------------------
 1 file changed, 19 insertions(+), 18 deletions(-)

(limited to 'kernel/trampoline.S')

diff --git a/kernel/trampoline.S b/kernel/trampoline.S
index fabaaf9..2ce4886 100644
--- a/kernel/trampoline.S
+++ b/kernel/trampoline.S
@@ -8,6 +8,10 @@
 	# kernel.ld causes this to be aligned
         # to a page boundary.
         #
+
+#include "riscv.h"
+#include "memlayout.h"
+
 	.section trampsec
 .globl trampoline
 trampoline:
@@ -20,14 +24,16 @@ uservec:
         # in supervisor mode, but with a
         # user page table.
         #
-        # sscratch points to where the process's p->trapframe is
-        # mapped into user space, at TRAPFRAME.
-        #
-        
-	# swap a0 and sscratch
-        # so that a0 is TRAPFRAME
-        csrrw a0, sscratch, a0
 
+        # save user a0 in sscratch so
+        # a0 can be used to get at TRAPFRAME.
+        csrw sscratch, a0
+
+        # each process has a separate p->trapframe memory area,
+        # but it's mapped to the same virtual address
+        # (TRAPFRAME) in every process.
+        li a0, TRAPFRAME
+        
         # save the user registers in TRAPFRAME
         sd ra, 40(a0)
         sd sp, 48(a0)
@@ -86,20 +92,15 @@ uservec:
 
 .globl userret
 userret:
-        # userret(TRAPFRAME, pagetable)
+        # userret(pagetable)
         # switch from kernel to user.
-        # usertrapret() calls here.
-        # a0: TRAPFRAME, in user page table.
-        # a1: user page table, for satp.
+        # a0: user page table, for satp.
 
         # switch to the user page table.
-        csrw satp, a1
+        csrw satp, a0
         sfence.vma zero, zero
 
-        # put the saved user a0 in sscratch, so we
-        # can swap it with our a0 (TRAPFRAME) in the last step.
-        ld t0, 112(a0)
-        csrw sscratch, t0
+        li a0, TRAPFRAME
 
         # restore all but a0 from TRAPFRAME
         ld ra, 40(a0)
@@ -133,8 +134,8 @@ userret:
         ld t5, 272(a0)
         ld t6, 280(a0)
 
-	# restore user a0, and save TRAPFRAME in sscratch
-        csrrw a0, sscratch, a0
+	# restore user a0
+        ld a0, 112(a0)
         
         # return to user mode and user pc.
         # usertrapret() set up sstatus and sepc.
-- 
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