Fix boot on newer qemu

kernel/kernelvec.S

@@ -85,6 +85,11 @@ kernelvec:
         // return to whatever we were doing in the kernel.
         sret
 
+// help: if you hit these, an unexpected exception/interrupt happened in M-mode.
+// use `info registers` in the qemu monitor to get the relevant CSRs.
+unexpected_exc: j unexpected_exc
+unexpected_int: j unexpected_int
+
         #
         # machine-mode timer interrupt.
         #
@@ -95,12 +100,21 @@ timervec:
         # scratch[0,8,16] : register save area.
         # scratch[24] : address of CLINT's MTIMECMP register.
         # scratch[32] : desired interval between interrupts.
-        
+
         csrrw a0, mscratch, a0
         sd a1, 0(a0)
         sd a2, 8(a0)
         sd a3, 16(a0)
 
+        csrr a1, mcause
+        // we should not get exceptions in M-mode. if we do, something went
+        // very wrong and we should explicitly jump to an infinite loop for the
+        // purpose.
+        bgez a1, unexpected_exc
+        li a2, (1<<63 | 7)
+        // likewise for any interrupts that are not machine timer interrupts.
+        bne a1, a2, unexpected_int
+
         # schedule the next timer interrupt
         # by adding interval to mtimecmp.
         ld a1, 24(a0) # CLINT_MTIMECMP(hart)
@@ -110,7 +124,7 @@ timervec:
         sd a3, 0(a1)
 
         # raise a supervisor software interrupt.
-	li a1, 2
+        li a1, 2
         csrw sip, a1
 
         ld a3, 16(a0)

kernel/riscv.h

@@ -38,6 +38,29 @@ w_mepc(uint64 x)
   asm volatile("csrw mepc, %0" : : "r" (x));
 }
 
+// physical memory protection CSRs
+#define PMP_R (1L << 0)
+#define PMP_W (1L << 1)
+#define PMP_X (1L << 2)
+// naturally aligned power of two
+#define PMP_MATCH_NAPOT (3L << 3)
+
+// we only implement accessing one PMP register
+
+// write to the first 8 PMP configuration registers
+static inline void
+w_pmpcfg0(uint64 x)
+{
+  asm volatile("csrw pmpcfg0, %0" : : "r" (x));
+}
+
+// write to the address for PMP region 0
+static inline void
+w_pmpaddr0(uint64 x)
+{
+  asm volatile("csrw pmpaddr0, %0" : : "r" (x));
+}
+
 // Supervisor Status Register, sstatus
 
 #define SSTATUS_SPP (1L << 8)  // Previous mode, 1=Supervisor, 0=User

kernel/start.c

@@ -7,6 +7,8 @@
 void main();
 void timerinit();
 
+static void pmpinit();
+
 // entry.S needs one stack per CPU.
 __attribute__ ((aligned (16))) char stack0[4096 * NCPU];
 
@@ -45,10 +47,36 @@ start()
   int id = r_mhartid();
   w_tp(id);
 
+  // allow access to all physical memory by S mode
+  pmpinit();
+
   // switch to supervisor mode and jump to main().
   asm volatile("mret");
 }
 
+// configures the pmp registers trivially so we can boot. it is not permitted
+// to jump to S mode without having these configured as instruction fetch will
+// fail, however we do not actually use them for protection in xv6, so we just
+// need to put something trivial there.
+//
+// see section 3.6.1 "Physical Memory Protection CSRs" in the RISC-V privileged
+// specification (v20190608)
+//
+// "If no PMP entry matches an M-mode access, the access succeeds. If no PMP
+// entry matches an S-mode or U-mode access, but at least one PMP entry is
+// implemented, the access fails." (3.6.1)
+static void
+pmpinit()
+{
+  // see figure 3.27 "PMP address register format, RV64" and table 3.10 "NAPOT
+  // range encoding in PMP address and configuration registers" in the RISC-V
+  // privileged specification
+  // we set the bits such that this matches any 56-bit physical address
+  w_pmpaddr0((~0ULL) >> 10);
+  // then we allow the access
+  w_pmpcfg0(PMP_R | PMP_W | PMP_X | PMP_MATCH_NAPOT);
+}
+
 // set up to receive timer interrupts in machine mode,
 // which arrive at timervec in kernelvec.S,
 // which turns them into software interrupts for