feat(riscv-rt): dedicated trap regions

Placed behind the `region-trap` feature flag.

Author: gibbz00

.github/workflows/tests.yaml

@@ -55,6 +55,8 @@ jobs:
         run: RUSTFLAGS="-C link-arg=-Tmemory.x -C link-arg=-Tlink.x" cargo build --package tests-build --target ${{ matrix.target }} --example ${{ matrix.example }} --features device
       - name: Build (include memory.x)
         run: RUSTFLAGS="-C link-arg=-Tdevice.x -C link-arg=-Tlink.x" cargo build --package tests-build --target ${{ matrix.target }} --example ${{ matrix.example }} --features memory
+      - name: Build (include memory.x, use trap region)
+        run: RUSTFLAGS="-C link-arg=-Tdevice.x -C link-arg=-Tlink.x" cargo build --package tests-build --target ${{ matrix.target }} --example ${{ matrix.example }} --features memory,trap-region
       - name: Build (include device.x and memory.x)
         run: RUSTFLAGS="-C link-arg=-Tlink.x" cargo build --package tests-build --target ${{ matrix.target }} --example ${{ matrix.example }} --features device,memory
 

riscv-rt/Cargo.toml

@@ -42,3 +42,4 @@ no-interrupts = []
 no-exceptions = []
 device = []
 memory = []
+trap-region = []

riscv-rt/link.x.in

@@ -127,6 +127,16 @@ PROVIDE(_max_hart_id = 0);
 PROVIDE(_hart_stack_size = SIZEOF(.stack) / (_max_hart_id + 1));
 PROVIDE(_heap_size = 0);
 
+{% set trap_regions %}
+. = ALIGN(4);
+KEEP(*(.trap.vector));   /* for _trap_vector (vectored mode only) */
+KEEP(*(.trap.start));    /* for _start_trap routine */
+KEEP(*(.trap.start.*));  /* for _start_INTERRUPT_trap routines (vectored mode only) */
+KEEP(*(.trap.continue)); /* for _continue_trap routine (vectored mode only) */
+KEEP(*(.trap.rust));     /* for _start_trap_rust Rust function */
+KEEP(*(.trap .trap.*));  /* Other .trap symbols at the end */
+{% endset %}
+
 SECTIONS
 {
   .text.dummy (NOLOAD) :
@@ -143,14 +153,9 @@ SECTIONS
     /* point of the program. */
     KEEP(*(.init));
 
-    . = ALIGN(4);
-    KEEP(*(.trap.vector));   /* for _trap_vector (vectored mode only) */
-    KEEP(*(.trap.start));    /* for _start_trap routine */
-    KEEP(*(.trap.start.*));  /* for _start_INTERRUPT_trap routines (vectored mode only) */
-    KEEP(*(.trap.continue)); /* for _continue_trap routine (vectored mode only) */
-    KEEP(*(.trap.rust));     /* for _start_trap_rust Rust function */
-    KEEP(*(.trap .trap.*));  /* Other .trap symbols at the end */
-
+    {% if not contains(cfg.feature, "trap-region") %}
+    {{ trap_regions }}
+    {% endif %}
     *(.text.abort);
     *(.text .text.*);
 
@@ -173,6 +178,13 @@ SECTIONS
     __erodata = .;
   } > REGION_RODATA
 
+  {% if contains(cfg.feature, "trap-region") %}
+  .trap : ALIGN(4)
+  {
+    {{ trap_regions }}
+  } > REGION_TRAP
+  {% endif %}
+
   .data : ALIGN({{ cfg.arch_width }})
   {
     . = ALIGN({{ cfg.arch_width }});

riscv-rt/src/lib.rs

@@ -532,6 +532,17 @@
 //! because when booting from elf, U-boot passes `argc` and `argv`. This feature also implies `single-hart`.
 //! The only way to get boot-hart is through fdt, so other harts initialization is up to you.
 //!
+//! ## `trap-region`
+//!
+//! Adds a dedicated trap memory region, `REGION_TRAP`, for placing `.trap` sections into. Enabling
+//! this feature and adding `REGION_ALIAS("REGION_TRAP", RAM);` to `memory.x` would for example place
+//! it in `RAM`.
+//!
+//! `REGION_TEXT` is usually placed in flash memory with a cache in front. Having a separate trap
+//! memory region makes it possible to always store interrupt and exceptions handlers in RAM,
+//! effectively bypassing cache contention and variable trap latency at the cost of increased RAM
+//! usage.
+//!
 //! [attr-entry]: attr.entry.html
 //! [attr-exception]: attr.exception.html
 //! [attr-external-interrupt]: attr.external_interrupt.html

tests-build/Cargo.toml

@@ -8,6 +8,9 @@ panic-halt = "1.0"
 riscv = { path = "../riscv" }
 riscv-rt = { path = "../riscv-rt" }
 
+[build-dependencies]
+minilink = "0.2"
+
 [features]
 pre-init = ["riscv-rt/pre-init"]
 single-hart = ["riscv-rt/single-hart"]
@@ -16,3 +19,4 @@ device = ["riscv-rt/device"]
 memory = ["riscv-rt/memory"]
 no-exceptions = ["riscv-rt/no-exceptions"]
 no-interrupts = ["riscv-rt/no-interrupts"]
+trap-region = ["riscv-rt/trap-region"]

tests-build/build.rs

@@ -1,22 +1,4 @@
-use std::{env, fs::File, io::Write, path::PathBuf};
-
 fn main() {
-    // Put device.x somewhere the linker can find it
-    let out = &PathBuf::from(env::var_os("OUT_DIR").unwrap());
-    File::create(out.join("device.x"))
-        .unwrap()
-        .write_all(include_bytes!("device.x"))
-        .unwrap();
-    println!("cargo:rustc-link-search={}", out.display());
-    println!("cargo:rerun-if-changed=device.x");
-
-    // Put memory.x somewhere the linker can find it
-    File::create(out.join("memory.x"))
-        .unwrap()
-        .write_all(include_bytes!("memory.x"))
-        .unwrap();
-    println!("cargo:rustc-link-search={}", out.display());
-    println!("cargo:rerun-if-changed=memory.x");
-
-    println!("cargo:rerun-if-changed=build.rs");
+    minilink::register_template("device.x", "device.x");
+    minilink::register_template("memory.x", "memory.x");
 }

tests-build/memory.x

@@ -10,3 +10,7 @@ REGION_ALIAS("REGION_DATA", RAM);
 REGION_ALIAS("REGION_BSS", RAM);
 REGION_ALIAS("REGION_HEAP", RAM);
 REGION_ALIAS("REGION_STACK", RAM);
+
+{% if contains(cfg.feature, "trap-region") %}
+REGION_ALIAS("REGION_TRAP", RAM);
+{% endif %}