takes one uart input interrupt, then panics

memlayout.h

@@ -13,9 +13,12 @@
 // end -- start of kernel page allocation area
 // PHYSTOP -- end RAM used by the kernel
 
-// registers start here in physical memory.
+// qemu puts UART registers here in physical memory.
 #define UART0 0x10000000L
 
+// qemu puts programmable interrupt controller here.
+#define PLIC 0x0c000000L
+
 #define RAMDISK 0x88000000
 
 // the kernel expects there to be RAM

proc.c

@@ -366,6 +366,22 @@ scheduler(void)
     // XXX riscv
     //sti();
 
+    if(0){ uint x = * (uint*) 0xc001000;
+      if(x != 0){
+        printf("pending %x\n", x);
+      }
+      x = *(uint*)0xc001004;
+      if(x != 0)
+        printf("pending %x\n", x);
+    }
+
+    if(0){
+      uint uartgetc(void);
+      uint x = uartgetc();
+      if(x != 0)
+        printf("%x ", x);
+    }
+    
     // Loop over process table looking for process to run.
     acquire(&ptable.lock);
     for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){

riscv.h

@@ -30,7 +30,11 @@ w_mepc(uint64 x)
 
 // Supervisor Status Register, sstatus
 
-#define SSTATUS_SPP (1L << 8) // 1=Supervisor, 0=User
+#define SSTATUS_SPP (1L << 8)  // Previous mode, 1=Supervisor, 0=User
+#define SSTATUS_SPIE (1L << 5) // Supervisor Previous Interrupt Enable
+#define SSTATUS_UPIE (1L << 4) // User Previous Interrupt Enable
+#define SSTATUS_SIE (1L << 1)  // Supervisor Interrupt Enable
+#define SSTATUS_UIE (1L << 0)  // User Interrupt Enable
 
 static inline uint64
 r_sstatus()
@@ -46,6 +50,33 @@ w_sstatus(uint64 x)
   asm("csrw sstatus, %0" : : "r" (x));
 }
 
+// Supervisor Interrupt Pending
+static inline uint64
+r_sip()
+{
+  uint64 x;
+  asm("csrr %0, sip" : "=r" (x) );
+  return x;
+}
+
+// Supervisor Interrupt Enable
+#define SIE_SEIE (1L << 9) // external
+#define SIE_STIE (1L << 5) // timer
+#define SIE_SSIE (1L << 1) // software
+static inline uint64
+r_sie()
+{
+  uint64 x;
+  asm("csrr %0, sie" : "=r" (x) );
+  return x;
+}
+
+static inline void 
+w_sie(uint64 x)
+{
+  asm("csrw sie, %0" : : "r" (x));
+}
+
 // machine exception program counter, holds the
 // instruction address to which a return from
 // exception will go.
@@ -147,6 +178,29 @@ r_stval()
   return x;
 }
 
+// enable interrupts
+static inline void
+intr_on()
+{
+  w_sie(r_sie() | SIE_SEIE | SIE_STIE | SIE_SSIE);
+  w_sstatus(r_sstatus() | SSTATUS_SIE);
+}
+
+// disable interrupts
+static inline void
+intr_off()
+{
+  w_sstatus(r_sstatus() & ~SSTATUS_SIE);
+}
+
+// are interrupts enabled?
+static inline int
+intr_get()
+{
+  uint64 x = r_sstatus();
+  return (x & SSTATUS_SIE) != 0;
+}
+
 #define PGSIZE 4096 // bytes per page
 #define PGSHIFT 12  // bits of offset within a page
 

trap.c

@@ -43,6 +43,19 @@ usertrap(void)
   // save user program counter.
   p->tf->epc = r_sepc();
 
+  // PLIC setup
+  // qemu makes UART0 be interrupt number 10.
+  int irq = 10;
+  // set uart's priority to be non-zero (otherwise disabled).
+  *(uint*)(0x0c000000L + irq*4) = 1;
+  // set uart's enable bit for hart 0 s-mode. 
+  *(uint*)0x0c002080 = (1 << irq);
+
+  // hart 0 S-mode priority threshold.
+  *(uint*)0x0c201000 = 0;
+
+  intr_on();
+  
   if(r_scause() == 8){
     // system call
 

uart.c

@@ -1,4 +1,10 @@
+#include "types.h"
+#include "param.h"
 #include "memlayout.h"
+#include "riscv.h"
+#include "proc.h"
+#include "spinlock.h"
+#include "defs.h"
 
 //
 // qemu -machine virt has a 16550a UART
@@ -9,12 +15,12 @@
 //
 
 // address of one of the registers
-#define R(reg) ((unsigned int*)(UART0 + 4*(reg)))
+#define R(reg) ((volatile unsigned char *)(UART0 + reg))
 
 void
 uartinit(void)
 {
-  // disable interrupts
+  // disable interrupts -- IER
   *R(1) = 0x00;
 
   // special mode to set baud rate
@@ -30,8 +36,11 @@ uartinit(void)
   // and set word length to 8 bits, no parity.
   *R(3) = 0x03;
 
-  // reset and enable FIFOs.
+  // reset and enable FIFOs -- FCR.
   *R(2) = 0x07;
+
+  // enable receive interrupts -- IER.
+  *R(1) = 0x01;
 }
 
 void
@@ -40,9 +49,11 @@ uartputc(int c)
   *R(0) = c;
 }
 
-static int
+uint
 uartgetc(void)
 {
+  // XXX this isn't right, must check there's data in the FIFO.
+  return *R(0);
 }
 
 void

vm.c

@@ -30,6 +30,11 @@ kvminit()
   mappages(kernel_pagetable, UART0, PGSIZE,
            UART0, PTE_R | PTE_W);
 
+  // PLIC
+  mappages(kernel_pagetable, PLIC, 0x4000000,
+           PLIC, PTE_R | PTE_W);
+  
+
   // map kernel text executable and read-only.
   mappages(kernel_pagetable, KERNBASE, (uint64)etext-KERNBASE,
            KERNBASE, PTE_R | PTE_X);