[lowrisc-dev] Support for Fedora / compressed extension

Dr Jonathan Kimmitt jrrk2 at cam.ac.uk
Mon Apr 2 08:23:50 BST 2018


Dear Richard,

My mistake, lowRISC-v0.5 runs a lightweight source based distribution based on RISCV-poky, using our modified 1.9.1 kernel and device drivers which have not been upstreamed. It would be perfectly possible to compile Fedora without compression but this would just move the bottleneck to a different place which is that the emulator is over a hundred times slower than a modern PC, and limited to 128M of RAM. Even running yum takes more than this at times. Swapping is also slow because it relies on individual writes to SD-card with 4 bits @ 5MHz, which itself has internal latency. In addition network access is throttled to about 2 megabytes per second due to TCP stack and bus width considerations.

With the best will in the world most mainstream distributions have more or less abandoned the embedded market by preferring heavyweight python based system tools which trade off memory for performance (feel free to dispute this sweeping generalisation, I know apt can run on the iPhone). I am aware of course that Python has many benefits in robustness and foolproofing.

Our system can perfectly well run on FPGA boards with larger memory but we are targeting low cost boards for the benefit of students and researchers at the moment.

Sent from my iPhone

> On 1 Apr 2018, at 21:50, Richard W.M. Jones <rjones at redhat.com> wrote:
> 
>> On Sun, Apr 01, 2018 at 09:46:32PM +0100, Dr Jonathan Kimmitt wrote:
>> The version of Rocket used in lowRISC was forked from an old version
>> of Rocket before the compressed instructions were added. Because
>> of the security extensions we have added it would not be easy to
>> merge the pipeline from a newer Rocket. For the same reason
>> debugging via JTAG is not available. I have never tried the tool
>> chain under Fedora,[..]
> 
> I am more interested in trying Fedora *on* lowRISC v0.5, however that
> will not work because Fedora/RISC-V is compiled for RV64GC.  Is
> adding/merging compressed instructions planned?
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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