From d2e243b81c5aa300dd0b742b26a37d10ac5ea182 Mon Sep 17 00:00:00 2001
From: Ali Saidi <alisaidi@amazon.com>
Date: Mon, 9 May 2022 20:50:33 -0500
Subject: [PATCH] Add support for using the Arm v8.5-RNG

---
 src/asm.rs                        |   1 +
 src/asm/random.rs                 | 111 ++++++++++++++++++++++++++++++
 src/registers.rs                  |   2 +
 src/registers/id_aa64isar0_el1.rs |  38 ++++++++++
 4 files changed, 152 insertions(+)
 create mode 100644 src/asm/random.rs
 create mode 100644 src/registers/id_aa64isar0_el1.rs

diff --git a/src/asm.rs b/src/asm.rs
index 0e4a942..24ba839 100644
--- a/src/asm.rs
+++ b/src/asm.rs
@@ -9,6 +9,7 @@
 //! Wrappers around ARMv8-A instructions.
 
 pub mod barrier;
+pub mod random;
 
 /// The classic no-op
 #[inline(always)]
diff --git a/src/asm/random.rs b/src/asm/random.rs
new file mode 100644
index 0000000..4d91117
--- /dev/null
+++ b/src/asm/random.rs
@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+//
+// Copyright (c) 2022 Amazon.com, Inc. or its affiliates.
+//
+// Author(s):
+//   - Ali Saidi <alisaidi@amazon.com>
+
+
+/// Implement an interface for accessing Arm v8.5 RNG instructions.
+/// An empty struct is used to confirm that the system has the
+/// instructions available.
+/// # Example:
+/// ```no_run
+/// use cortex_a::asm::random::ArmRng;
+/// if let Some(rng) = ArmRng::new() {
+///    let rand_num = rng.rndr();
+/// }
+/// ```
+#[derive(Copy, Clone, Debug)]
+pub struct ArmRng;
+
+use crate::registers::ID_AA64ISAR0_EL1;
+use core::arch::asm;
+use tock_registers::interfaces::Readable;
+
+impl ArmRng {
+    /// Return an empty object that is used to gate calling
+    /// rndr and rndrss on discovery of the feature so each
+    /// call doesn't need to confirm it.
+    #[inline]
+    pub fn new() -> Option<Self> {
+        #[cfg(not(target_arch = "aarch64"))]
+        return None;
+
+        #[cfg(target_arch = "aarch64")]
+        if ID_AA64ISAR0_EL1.is_set(ID_AA64ISAR0_EL1::RNDR) {
+            Some(ArmRng)
+        } else {
+            None
+        }
+    }
+
+    /// Return an random number from the Arm v8.5 RNG.
+    /// This returns an option because the instruction can fail
+    /// (e.g. the entropy is exhausted or the RNG has failed.)
+    #[inline]
+    pub fn rndr(&self) -> Option<u64> {
+        let mut flags: u64;
+        let mut data: u64;
+
+        #[cfg(target_arch = "aarch64")]
+        unsafe {
+            asm!(
+                "mrs {o}, s3_3_c2_c4_0",
+                "mrs {f}, nzcv",
+                o = out(reg) data,
+                f = out(reg) flags,
+                options(nomem, nostack));
+        }
+        if cfg!(not(target_arch = "aarch64")) || flags != 0 {
+            None
+        } else {
+            Some(data)
+        }
+    }
+
+    /// Return an random number from the Arm v8.5 RNG after reseeding it
+    /// This returns an option because the instruction can fail
+    /// (e.g. the entropy is exhausted or the RNG has failed.)
+    #[inline]
+    pub fn rndrss(&self) -> Option<u64> {
+        let mut flags: u64;
+        let mut data: u64;
+
+        #[cfg(target_arch = "aarch64")]
+        unsafe {
+            asm!(
+                "mrs {o}, s3_3_c2_c4_1",
+                "mrs {f}, nzcv",
+                o = out(reg) data,
+                f = out(reg) flags,
+                options(nomem, nostack));
+        }
+
+        if cfg!(not(target_arch = "aarch64")) || flags != 0 {
+            None
+        } else {
+            Some(data)
+        }
+    }
+
+
+}
+
+
+#[cfg(all(test, target_os = "linux"))]
+mod tests {
+    use super::*;
+
+    #[test]
+    pub fn test_rndr() {
+        // This works on Linux from userspace since Linux emulatates
+        // the Arm ID registers on the userspace undef.
+        if let Some(rand) = ArmRng::new() {
+            assert!(rand.rndr().unwrap() != 0);
+            assert!(rand.rndrss().unwrap() != 0);
+        }
+    }
+
+}
+
diff --git a/src/registers.rs b/src/registers.rs
index b805942..8ad25a1 100644
--- a/src/registers.rs
+++ b/src/registers.rs
@@ -28,6 +28,7 @@ mod far_el2;
 mod fp;
 mod hcr_el2;
 mod id_aa64mmfr0_el1;
+mod id_aa64isar0_el1;
 mod lr;
 mod mair_el1;
 mod mair_el2;
@@ -78,6 +79,7 @@ pub use far_el2::FAR_EL2;
 pub use fp::FP;
 pub use hcr_el2::HCR_EL2;
 pub use id_aa64mmfr0_el1::ID_AA64MMFR0_EL1;
+pub use id_aa64isar0_el1::ID_AA64ISAR0_EL1;
 pub use lr::LR;
 pub use mair_el1::MAIR_EL1;
 pub use mair_el2::MAIR_EL2;
diff --git a/src/registers/id_aa64isar0_el1.rs b/src/registers/id_aa64isar0_el1.rs
new file mode 100644
index 0000000..9c61fe1
--- /dev/null
+++ b/src/registers/id_aa64isar0_el1.rs
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+//
+// Copyright (c) 2022 Amazon.com, Inc. or its affiliates.
+//
+// Author(s):
+//   - Ali Saidi <alisaidi@amazon.com>
+
+//! AArch64 Instruction Set Architecture Feature Register 0 - EL1
+//!
+//! Provides information about the implemented instruction set.
+
+use tock_registers::{interfaces::Readable, register_bitfields};
+
+register_bitfields! {u64,
+    pub ID_AA64ISAR0_EL1 [
+        /// Support for Random Number instructions in AArch64.
+        ///
+        /// 0000 No random number instructions are implemented
+        /// 0001 RNDR and RNDRSS are implemented
+        ///
+        /// All other values are reserved.
+        RNDR OFFSET(60) NUMBITS(4) [
+            Supported = 0b0001,
+            NotSupported = 0b0000
+        ],
+    ]
+}
+
+pub struct Reg;
+
+impl Readable for Reg {
+    type T = u64;
+    type R = ID_AA64ISAR0_EL1::Register;
+
+    sys_coproc_read_raw!(u64, "ID_AA64ISAR0_EL1", "x");
+}
+
+pub const ID_AA64ISAR0_EL1: Reg = Reg {};