zero_stream: Fix read callback calling convention and buffer memory region

1 files changed, 12 insertions, 2 deletions

diff --git a/examples/zero_stream/src/main.zig b/examples/zero_stream/src/main.zig
index 7147972..1536157 100644
--- a/examples/zero_stream/src/main.zig
+++ b/examples/zero_stream/src/main.zig
@@ -9,6 +9,15 @@ const Result = extern struct {
     status: usize,
 };
 
+// The buffer has to be a global variable
+// because a stack buffer would be mapped to different physical memory addresses
+// in the driver thread compared to the main thread due to paging
+// (because each thread has its own stack).
+// This will be fixed in a future kernel version because the same root cause
+// also prevents a distinct driver process from accessing the memory
+// (its attempt to modify the buffer would raise a page fault).
+var buf = [8]u8{ 1, 2, 3, 4, 5, 6, 7, 8 };
+
 export fn _start() void {
     main();
 }
@@ -64,7 +73,6 @@ pub fn main() void {
         : "a7"
     );
 
-    var buf = [8]u8{ 1, 2, 3, 4, 5, 6, 7, 8 };
     asm volatile (
         \\ li a7, 100009
         \\ ecall
@@ -86,7 +94,9 @@ pub fn main() void {
     );
 }
 
-pub fn read(buffer: []u8) Result {
+pub export fn read(ptr: [*]u8, len: usize) Result {
+    const buffer = ptr[0..len];
+
     for (buffer, 0..) |_, i| {
         buffer[i] = 0;
     }