mirror of
https://github.com/antonblanchard/microwatt.git
synced 2026-01-13 15:18:09 +00:00
9b3b57710a
This fixes two bugs in the flash invalidation of the icache. The first is that an instruction could get executed twice. The i-cache RAM is 2 instructions (64 bits) wide, so one read can supply results for 2 cycles. The fetch1 stage tells icache when the address is equal to the address of the previous cycle plus 4, and in cases where that is true, bit 2 of the address is 1, and the previous cycle was a cache hit, we just use the second word of the doubleword read from the cache RAM. However, the cache hit/miss logic also continues to operate, so in the case where the first word hits but the second word misses (because of an icache invalidation or a snoop occurring in the first cycle), we supply the instruction from the data previously read from the icache RAM but also stall fetch1 and start a cache reload sequence, and subsequently supply the second instruction again. This fixes the issue by inhibiting req_is_miss and stall_out when use_previous is true. The second bug is that if an icache invalidation occurs while reloading a line, we continue to reload the line, and make it valid when the reload finishes, even though some of the data may have been read before the invalidation occurred. This adds a new state STOP_RELOAD which we go to if an invalidation happens while we are in CLR_TAG or WAIT_ACK state. In STOP_RELOAD state we don't request any more reads from memory and wait for the reads we have previously requested to be acked, and then go to IDLE state. Data returned is still written to the icache RAM, but that doesn't matter because the line is invalid and is never made valid. Note that we don't have to worry about invalidations due to snooped writes while reloading a line, because the wishbone arbiter won't switch to another master once it has started sending our reload requests to memory. Thus a store to memory will either happen before any of our reads have got to memory, or after we have finished the reload (in which case we will no longer be in WAIT_ACK state). Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Microwatt
A tiny Open POWER ISA softcore written in VHDL 2008. It aims to be simple and easy to understand.
Simulation using ghdl
You can try out Microwatt/Micropython without hardware by using the ghdl simulator. If you want to build directly for a hardware target board, see below.
- Build micropython. If you aren't building on a ppc64le box you will need a cross compiler. If it isn't available on your distro grab the powerpc64le-power8 toolchain from https://toolchains.bootlin.com. You may need to set the CROSS_COMPILE environment variable to the prefix used for your cross compilers. The default is powerpc64le-linux-gnu-.
git clone https://github.com/micropython/micropython.git
cd micropython
cd ports/powerpc
make -j$(nproc)
cd ../../../
A prebuilt micropython image is also available in the micropython/ directory.
-
Microwatt uses ghdl for simulation. Either install this from your distro or build it. Microwatt requires ghdl to be built with the LLVM or gcc backend, which not all distros do (Fedora does, Debian/Ubuntu appears not to). ghdl with the LLVM backend is likely easier to build.
If building ghdl from scratch is too much for you, the microwatt Makefile supports using Docker or Podman.
-
Next build microwatt:
git clone https://github.com/antonblanchard/microwatt
cd microwatt
make
To build using Docker:
make DOCKER=1
and to build using Podman:
make PODMAN=1
- Link in the micropython image:
ln -s ../micropython/ports/powerpc/build/firmware.bin main_ram.bin
Or if you were using the pre-built image:
ln -s micropython/firmware.bin main_ram.bin
- Now run microwatt, sending debug output to /dev/null:
./core_tb > /dev/null
Synthesis on Xilinx FPGAs using Vivado
-
Install Vivado (I'm using the free 2019.1 webpack edition).
-
Setup Vivado paths:
source /opt/Xilinx/Vivado/2019.1/settings64.sh
- Install FuseSoC:
pip3 install --user -U fusesoc
Fedora users can get FuseSoC package via
sudo dnf copr enable sharkcz/danny
sudo dnf install fusesoc
- If this is your first time using fusesoc, initialize fusesoc. This is needed to be able to pull down fussoc library components referenced by microwatt. Run
fusesoc init
- Create a working directory and point FuseSoC at microwatt:
mkdir microwatt-fusesoc
cd microwatt-fusesoc
fusesoc library add microwatt /path/to/microwatt/
- Build using FuseSoC. For hello world (Replace nexys_video with your FPGA board such as --target=arty_a7-100): You may wish to ensure you have installed Digilent Board files or appropriate files for your board first.
fusesoc run --target=nexys_video microwatt --memory_size=16384 --ram_init_file=/path/to/microwatt/fpga/hello_world.hex
You should then be able to see output via the serial port of the board (/dev/ttyUSB1, 115200 for example assuming standard clock speeds). There is a know bug where initial output may not be sent - try the reset (not programming button) on your board if you don't see anything.
- To build micropython (currently requires 1MB of BRAM eg an Artix-7 A200):
fusesoc run --target=nexys_video microwatt
Testing
- A simple test suite containing random execution test cases and a couple of micropython test cases can be run with:
make -j$(nproc) check
Issues
This is functional, but very simple. We still have quite a lot to do:
- There are a few instructions still to be implemented
- Need to add caches and bypassing (in progress)
- Need to add supervisor state (in progress)