Fixes from lfcode.cariscv

PMP enforecement from Qemu breaks boot cycle. This fix is from
upstream repo here: https://github.com/lf-/xv6-riscv/

1 files changed, 10 insertions, 1 deletions

diff --git a/kernel/start.c b/kernel/start.c
index 1876680..2d89d90 100644
--- a/kernel/start.c
+++ b/kernel/start.c
@@ -44,7 +44,16 @@ start()
   // keep each CPU's hartid in its tp register, for cpuid().
   int id = r_mhartid();
   w_tp(id);
-
+  
+  // allow access to all physical memory by S mode
+    // 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);  
+  
   // switch to supervisor mode and jump to main().
   asm volatile("mret");
 }