adopt github PR98 (No need to store TRAPFRAME in sscratch register)

kernel/riscv.h

@@ -1,3 +1,5 @@
+#ifndef __ASSEMBLER__
+
 // which hart (core) is this?
 static inline uint64
 r_mhartid()
@@ -331,6 +333,10 @@ sfence_vma()
   asm volatile("sfence.vma zero, zero");
 }
 
+typedef uint64 pte_t;
+typedef uint64 *pagetable_t; // 512 PTEs
+
+#endif // __ASSEMBLER__
 
 #define PGSIZE 4096 // bytes per page
 #define PGSHIFT 12  // bits of offset within a page
@@ -361,6 +367,3 @@ sfence_vma()
 // Sv39, to avoid having to sign-extend virtual addresses
 // that have the high bit set.
 #define MAXVA (1L << (9 + 9 + 9 + 12 - 1))
-
-typedef uint64 pte_t;
-typedef uint64 *pagetable_t; // 512 PTEs

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.

kernel/trap.c

@@ -96,8 +96,9 @@ usertrapret(void)
   // we're back in user space, where usertrap() is correct.
   intr_off();
 
-  // send syscalls, interrupts, and exceptions to trampoline.S
-  w_stvec(TRAMPOLINE + (uservec - trampoline));
+  // send syscalls, interrupts, and exceptions to uservec in trampoline.S
+  uint64 trampoline_uservec = TRAMPOLINE + (uservec - trampoline);
+  w_stvec(trampoline_uservec);
 
   // set up trapframe values that uservec will need when
   // the process next re-enters the kernel.
@@ -121,11 +122,11 @@ usertrapret(void)
   // tell trampoline.S the user page table to switch to.
   uint64 satp = MAKE_SATP(p->pagetable);
 
-  // jump to trampoline.S at the top of memory, which 
+  // jump to userret in trampoline.S at the top of memory, which 
   // switches to the user page table, restores user registers,
   // and switches to user mode with sret.
-  uint64 fn = TRAMPOLINE + (userret - trampoline);
-  ((void (*)(uint64,uint64))fn)(TRAPFRAME, satp);
+  uint64 trampoline_userret = TRAMPOLINE + (userret - trampoline);
+  ((void (*)(uint64))trampoline_userret)(satp);
 }
 
 // interrupts and exceptions from kernel code go here via kernelvec,