PoC: Implement syscalls using PicoRV32 interrupts #305

agren · 2024-12-09T12:18:23Z

Description

This is a proof of concept of using PicoRV32 interrupts to implement syscalls.
The idea is to let in app execute system calls by raising an interrupt. The
following sequence describes a syscall call:

App raises interrupt by writing to a raise-interrupt-address
Interrupt handler get executed
Syscall gets handled
Interrupt handler returns to app by calling PicoRV32-specific
instruction retirq.

One interrupt is added: IRQ31. An interrupt is triggered by writing
to a specific memory area. Triggering an interrupt hands over execution to the interrupt handler in firmware ROM (address 0x10).
The PicoRV32 eoi signal is used to make certain resources, which are otherwise unavailable in app mode, available to the interrupt handler.

Four example firmwares are available to demonstrate interrupt handling:

hw/application_fpga/fw/irqpoc/start.S
hw/application_fpga/fw/irqpoc_led_toggle/start.S
hw/application_fpga/fw/irq_poc_with_app/start.S
hw/application_fpga/fw/irqpoc_c_example/start.S

Added so far:

Enable PicoRV32 interrupts
Allow access to resources during syscall interrupt
- Executing from firmware ROM allowed
- Firmware RAM available
- SPI accessible
Lock down resources only available in firmware mode
Example firmware with syscall implementation in C (see irqpoc_c_example)
Example app calling of syscalls from C

This draft PR is part of investigating #234.

dehanj · 2024-12-18T14:58:35Z

hw/application_fpga/core/tk1/rtl/tk1.v

@@ -291,7 +293,7 @@ module tk1 #(
      gpio2_reg[1] <= gpio2_reg[0];

      if (system_mode_we) begin
-        system_mode_reg <= 1'h1;
+        system_mode_reg <= system_mode_new;


I think we should keep system_mode_reg <= 1'h1;

Then we can remove system_mode_new completely. Since what I can see we never return to system_mode_reg = 0.

A proof-of-concept of enabling PicoRV32 interrupts. Two interrupt sources, which can be triggered by writes to memory addresses, are added. The design has only been simulated, not run on hardware. Synthesis: Ice40 LC utilization is 93% (4934/5280) when built using tkey-builder:4 Simulation: A `tb_application_fpga_irqpoc` target is added. Running `make tb_application_fpga_irqpoc` creates `tb_application_fpga_sim.fst` which can be inspected in GTKWave or Surfer. Firmware: A simple firmware is added in `fw/irqpoc`. It enables both interrupts and triggers each interrupt once. Custom PicoRV32 instructions are located in `custom_ops.S`. It is imported from upstream PicoRV32 commit: YosysHQ/picorv32@70f3c33

Add example firmware for demoing interrupts on Tkey hardware.

Instead of manually switching to app mode using the system mode register, app mode will be enabled when executing outside of firmware ROM. Co-authored-by: Mikael Ågren <mikael@tillitis.se>

Only allow executing from ROM when in one of the following execution contexts: - Firmware mode - IRQ_SYSCALL_LO - IRQ_SYSCALL_HI Co-authored-by: Daniel Jobson <jobson@tillitis.se>

Adds a basic example firmware that copies an app to app RAM. The app triggers syscall interrupts and tries to execute ROM code from app mode. A make target (`tb_application_fpga_irqpoc_with_app`) that simulates a Tkey running the firmware is added.

Allow FW RAM access only in the following execution contexts: - Firmware mode - IRQ_SYSCALL_HI Input port `system_mode` of the `fw_ram` module is replaced with an enable port. Since access to FW RAM not longer depend only on system_mode

…ed in C App is embedded in firmware and is loaded into app RAM when firmware starts. App continuously calls SET_LED syscalls. Simulation: `make tb_application_fpga_irqpoc_c_example`

Co-authored-by: Mikael Ågren <mikael@tillitis.se>

…is set After the first time system_mode is set to one, the assets will no longer be read- or writeable, even if system_mode is set to zero at a later syscall. This is to make sure syscalls does not have the same privilege as the firmware has at first boot. We need to monitor when system_mode is set to one, otherwise we might accedentially lock the assets before actually leaving firmware, for example if firmware would use a function set in any of the registers used in system_mode_ctrl. Co-authored-by: Mikael Ågren <mikael@tillitis.se>

Removing IRQ30 since it us no longer exist

Keep WE and CS high for one clock cycle instead of two. To avoid writing the same address twice.

Always display errors to make them easy to find and troubleshoot.

Fixing tests that broke when adding interrupt based syscalls

Removing the blake2s test since the blake2s registers are removed.

Fix test1. It broke while implementing interrupt based syscalls. Instead of writing to ADDR_SYSTEM_MODE_CTRL, app mode is now entered automatically when executing outside of ROM.

Fix test10. It broke while implementing interrupt based syscalls. Cleaning up after the previous test. We reset the memory bus to a known idle state. We also reset the DUT to make the SPI master visible.

…calls Checks availability of: - CDI - UDI - RAM - SPI

Adds: - SYSCALL_RESET - SYSCALL_SET_LED

Removing the blake2s test since the possibility for the firmware to expose a blake2s function to the app has been removed.

App mode can no longer be controlled from software. So the tests have to run from firmware RAM.

agren requested review from dehanj, mchack-work and jthornblad December 9, 2024 12:18

agren force-pushed the syscall_picorv32_irq branch from 54af78a to 713a9e0 Compare December 9, 2024 13:03

agren mentioned this pull request Dec 10, 2024

Hardware syscall support #234

Open

agren force-pushed the syscall_picorv32_irq branch 5 times, most recently from 4afcea2 to 6023d97 Compare December 17, 2024 17:16

dehanj reviewed Dec 18, 2024

View reviewed changes

mchack-work mentioned this pull request Jan 21, 2025

Idea: multiple inputs to blake2s in firmware #157

Closed

mchack-work linked an issue Jan 21, 2025 that may be closed by this pull request

Hardware syscall support #234

Open

agren force-pushed the syscall_picorv32_irq branch 7 times, most recently from c40f63c to c8f43bb Compare February 4, 2025 08:35

agren added 2 commits February 4, 2025 12:25

PoC: Add LED toggling interrupt example

d36e9c9

Add example firmware for demoing interrupts on Tkey hardware.

agren force-pushed the syscall_picorv32_irq branch from c8f43bb to 2a11b76 Compare February 4, 2025 11:32

dehanj and others added 7 commits February 7, 2025 12:54

PoC: Automatically control system_mode in hardware

2d762fa

Instead of manually switching to app mode using the system mode register, app mode will be enabled when executing outside of firmware ROM. Co-authored-by: Mikael Ågren <mikael@tillitis.se>

PoC: Remove Blake2s register

b53666e

PoC: Trap when executing from ROM in app mode

e4d19e8

Only allow executing from ROM when in one of the following execution contexts: - Firmware mode - IRQ_SYSCALL_LO - IRQ_SYSCALL_HI Co-authored-by: Daniel Jobson <jobson@tillitis.se>

PoC: Control access to FW RAM

62dba7c

Allow FW RAM access only in the following execution contexts: - Firmware mode - IRQ_SYSCALL_HI Input port `system_mode` of the `fw_ram` module is replaced with an enable port. Since access to FW RAM not longer depend only on system_mode

PoC: Add example firmware with embedded that calls syscalls implement…

4877e0a

…ed in C App is embedded in firmware and is loaded into app RAM when firmware starts. App continuously calls SET_LED syscalls. Simulation: `make tb_application_fpga_irqpoc_c_example`

PoC: Deny access to the SPI master in app mode

ecdbb25

Co-authored-by: Mikael Ågren <mikael@tillitis.se>

agren and others added 23 commits February 7, 2025 12:54

PoC: Remove low privilege syscall

7f34f5d

PoC: Remove IRQ30 from fw/irqpoc

0520292

Removing IRQ30 since it us no longer exist

PoC: Remove IRQ30 from fw/irqpoc_led_toggle

14f266e

Removing IRQ30 since it us no longer exist

PoC: Remove IRQ30 from fw/irqpoc_with_app

82d408f

Removing IRQ30 since it us no longer exist

PoC: Remove IRQ30 from fw/irqpoc_c_example

7f95e09

Removing IRQ30 since it us no longer exist

PoC: tb: Write data only once per call to write_word() in tb_tk1

3269d25

Keep WE and CS high for one clock cycle instead of two. To avoid writing the same address twice.

tb: Display errors in tb_tk1 even if DEBUG is 0

93a74bc

Always display errors to make them easy to find and troubleshoot.

PoC: tb: Update tk1 test bench with new ports

007aa69

Fixing tests that broke when adding interrupt based syscalls

PoC: tb: Remove tb_tk1 blake2s test

a646c8b

Removing the blake2s test since the blake2s registers are removed.

PoC: tb: Add fetch instruction helper task to tb_tk1

5c56304

PoC: tb: Fix tb_tk1 test3 (CDI)

6c2a7ef

Fix test1. It broke while implementing interrupt based syscalls. Instead of writing to ADDR_SYSTEM_MODE_CTRL, app mode is now entered automatically when executing outside of ROM.

PoC: tb: Fix tb_tk1 test5 (APP_START/APP_SIZE)

8c6e4a3

Fix test1. It broke while implementing interrupt based syscalls. Instead of writing to ADDR_SYSTEM_MODE_CTRL, app mode is now entered automatically when executing outside of ROM.

PoC: tb: Fix tb_tk1 test10 (SPI loop back test)

97005bb

Fix test10. It broke while implementing interrupt based syscalls. Cleaning up after the previous test. We reset the memory bus to a known idle state. We also reset the DUT to make the SPI master visible.

PoC: tb: Expand existing tests with access checks in app mode and sys…

e851efd

…calls Checks availability of: - CDI - UDI - RAM - SPI

PoC: Add experimental syscalls to firmware

926f3c6

Adds: - SYSCALL_RESET - SYSCALL_SET_LED

PoC: testfw: Remove blake2s test

1a505f4

Removing the blake2s test since the possibility for the firmware to expose a blake2s function to the app has been removed.

PoC: testfw: Break out tests running in app mode into separate app

d1b8b6e

App mode can no longer be controlled from software. So the tests have to run from firmware RAM.

PoC: fw: Import spi.[ch] and flash.[ch]

ea7f710

PoC: Integrate spi and flash code

52694b5

PoC: testfw: Check that SPI flash is available in firmware mode

3665c3e

PoC: testapp: Call syscall accessing SPI flash

f34b4c3

PoC: testapp: Call reset syscall

c0a9819

agren force-pushed the syscall_picorv32_irq branch from 2a11b76 to c0a9819 Compare February 7, 2025 11:55

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PoC: Implement syscalls using PicoRV32 interrupts #305

PoC: Implement syscalls using PicoRV32 interrupts #305

agren commented Dec 9, 2024 •

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dehanj Dec 18, 2024

PoC: Implement syscalls using PicoRV32 interrupts #305

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