add a configuration that works

for some reason a debug-print is essential for the system to run
stably. It smells like we're violating some minimum time between
accesses of SPIM interface, or else we're encoutnering some weird
optimization bug.

services/xous-swapper/Cargo.toml

@@ -38,4 +38,4 @@ swap-userspace-testing = []
 
 debug-print = []
 debug-verbose = []
-default = ["debug-print", "debug-verbose"]
+default = ["debug-print"]

services/xous-swapper/src/main.rs

@@ -460,7 +460,7 @@ fn swap_handler(
     let ss = sss.inner.as_mut().expect("Shared state should be initialized");
 
     let op: Option<KernelOp> = FromPrimitive::from_usize(opcode);
-    #[cfg(feature = "debug-print")]
+    #[cfg(feature = "debug-print-verbose")]
     writeln!(DebugUart {}, "got Opcode: {:?}", op).ok();
     match op {
         Some(KernelOp::WriteToSwap) => {
@@ -483,15 +483,6 @@ fn swap_handler(
             let pid = a2 as u8;
             let vaddr_in_pid = a3;
             let vaddr_in_swap = a4;
-            #[cfg(feature = "debug-print")]
-            writeln!(
-                DebugUart {},
-                "RFS PID{}, vaddr_pid {:x}, vaddr_swap {:x}",
-                pid,
-                vaddr_in_pid,
-                vaddr_in_swap
-            )
-            .ok();
             // walk the PT to find the swap data, and remove it from the swap PT
             let paddr_in_swap = match ss.pt_walk(pid as u8, vaddr_in_pid, true) {
                 Some(paddr) => paddr,
@@ -501,8 +492,19 @@ fn swap_handler(
                     panic!("Couldn't resolve swapped data. Was the page actually swapped?")
                 }
             };
-            #[cfg(feature = "debug-verbose")]
-            writeln!(DebugUart {}, "   resolved paddr_in_swap {:x}", paddr_in_swap).ok();
+            // for some reason, `paddr_in_swap` must be printed for the routine to not crash. Or the delay
+            // after `pt_walk` is necessary. Either way, it's spooky. I wonder if there isn't some minimum
+            // time between reads from the SPIM that we're violating??
+            #[cfg(feature = "debug-print")]
+            writeln!(
+                DebugUart {},
+                "RFS PID{}, vaddr_pid {:x}, vaddr_swap {:x}, paddr {:x}",
+                pid,
+                vaddr_in_pid,
+                vaddr_in_swap,
+                paddr_in_swap
+            )
+            .ok();
             // clear the used bit in swap
             ss.sct.counts[paddr_in_swap / PAGE_SIZE] &= !loader::FLG_SWAP_USED;