From 2001632142a11d9a940eee7e3bfbe7c0186fe6ce Mon Sep 17 00:00:00 2001
From: Andrew Waterman <waterman@cs.berkeley.edu>
Date: Mon, 2 May 2016 14:30:48 -0700
Subject: Don't use tohost/fromhost registers; communicate with host via memory

---
 machine/mentry.S | 14 +++-----------
 machine/mtrap.c  | 31 ++++++++++++++++++-------------
 2 files changed, 21 insertions(+), 24 deletions(-)

diff --git a/machine/mentry.S b/machine/mentry.S
index bbc1c2a..4895227 100644
--- a/machine/mentry.S
+++ b/machine/mentry.S
@@ -18,11 +18,9 @@ trap_table:
   .word mcall_trap
   .word bad_trap
   .word bad_trap
-#define HTIF_INTERRUPT_VECTOR 12
-  .word htif_interrupt
-#define SOFTWARE_INTERRUPT_VECTOR 13
+#define SOFTWARE_INTERRUPT_VECTOR 12
   .word software_interrupt
-#define TRAP_FROM_MACHINE_MODE_VECTOR 14
+#define TRAP_FROM_MACHINE_MODE_VECTOR 13
   .word __trap_from_machine_mode
 
   .option norvc
@@ -67,16 +65,10 @@ trap_vector:
 1:
   # Is it an IPI?
   li a0, IRQ_M_SOFT * 2
-  bne a0, a1, 1f
+  bne a0, a1, .Lunhandleable_trap
   li a1, SOFTWARE_INTERRUPT_VECTOR
   j .Lhandle_trap_in_machine_mode
 
-1:
-  # By process of elimination, it must be an HTIF interrupt.
-  li a0, IRQ_HOST * 2
-  bne a0, a1, .Lunhandleable_trap
-  li a1, HTIF_INTERRUPT_VECTOR
-
 .Lhandle_trap_in_machine_mode:
   # Preserve the registers.  Compute the address of the trap handler.
   STORE ra, 1*REGBYTES(sp)
diff --git a/machine/mtrap.c b/machine/mtrap.c
index 4087a67..15ecf8f 100644
--- a/machine/mtrap.c
+++ b/machine/mtrap.c
@@ -7,6 +7,9 @@
 #include <stdarg.h>
 #include <stdio.h>
 
+volatile uint64_t tohost __attribute__((aligned(64)));
+volatile uint64_t fromhost __attribute__((aligned(64)));
+
 void __attribute__((noreturn)) bad_trap()
 {
   die("machine mode: unhandlable trap %d @ %p", read_csr(mcause), read_csr(mepc));
@@ -19,31 +22,33 @@ static uintptr_t mcall_hart_id()
 
 static void request_htif_keyboard_interrupt()
 {
-  uintptr_t old_tohost = swap_csr(mtohost, TOHOST_CMD(1, 0, 0));
-  assert(old_tohost == 0);
+  assert(tohost == 0);
+  tohost = TOHOST_CMD(1, 0, 0);
 }
 
-void htif_interrupt()
+static void htif_interrupt()
 {
-  uintptr_t fromhost = swap_csr(mfromhost, 0);
   // we should only be interrupted by keypresses
-  if (!(FROMHOST_DEV(fromhost) == 1 && FROMHOST_CMD(fromhost) == 0))
+  uint64_t fh = fromhost;
+  if (!(FROMHOST_DEV(fh) == 1 && FROMHOST_CMD(fh) == 0))
     die("unexpected htif interrupt");
-  HLS()->console_ibuf = 1 + (uint8_t)FROMHOST_DATA(fromhost);
+  HLS()->console_ibuf = 1 + (uint8_t)FROMHOST_DATA(fh);
+  fromhost = 0;
   set_csr(mip, MIP_SSIP);
 }
 
 static void do_tohost_fromhost(uintptr_t dev, uintptr_t cmd, uintptr_t data)
 {
-  while (swap_csr(mtohost, TOHOST_CMD(dev, cmd, data)) != 0)
-    if (read_csr(mfromhost))
+  while (tohost)
+    if (fromhost)
       htif_interrupt();
+  tohost = TOHOST_CMD(dev, cmd, data);
 
   while (1) {
-    uintptr_t fromhost = read_csr(mfromhost);
-    if (fromhost) {
-      if (FROMHOST_DEV(fromhost) == dev && FROMHOST_CMD(fromhost) == cmd) {
-        write_csr(mfromhost, 0);
+    uint64_t fh = fromhost;
+    if (fh) {
+      if (FROMHOST_DEV(fh) == dev && FROMHOST_CMD(fh) == cmd) {
+        fromhost = 0;
         break;
       }
       htif_interrupt();
@@ -66,7 +71,7 @@ static uintptr_t mcall_htif_syscall(uintptr_t magic_mem)
 void poweroff()
 {
   while (1)
-    write_csr(mtohost, 1);
+    tohost = 1;
 }
 
 void putstring(const char* s)
-- 
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