Questions for OpenSBI to support position independent execution

[Anup Patel]

On Fri, Jan 22, 2021 at 9:41 AM Vincent Chen <vincent.chen at sifive.com> wrote:
>
> Hi,
> For the current implementation,  if we configure the OpenSBI as the
> generic platform, we can execute the same OpenSBI image on the
> different platforms. It eliminates many troubles in use. However, I
> found there is an assumption that the OpenSBI only can run at the same
> address (default: 0x80000000). If users want to change the OpenSBI
> running address due to platform memory layout or other reasons, users
> need to change the FW_TEXT_START and recompile the OpenSBI. In this
> case, users still need to prepare multiple images for different
> platforms. In order to overcome this issue, I am evaluating the
> feasibility of enabling OpenSBI to support position-independent
> execution. If someone has any concerns about this issue or any
> suggestions, please let me know. Thanks

The proposal for position-independent OpenSBI firmwares has been
suggested in the past. Our only concern with position-independent
code is additional instructions added in hot-path which can affect
performances because all global variables and global functions will
now be referred via GOT.

We need a better analysis about performance impact due to
position-independent OpenSBI firmwares.

>
> In addition, when I review the current implementation, I encounter a
> question shown below. After OpenSBI enables to support the
> position-independent execution, ideally, the code can run at an
> arbitrary place. Hence, I think the codes could be removed from
> firmware/fw_base.S, which are used to copy the code from the load
> address to the run address. Before removing them, I want to know the
> original purpose and usages of these codes, but I can't find them. If
> some can tell me the answer, I would appreciate it.

The self-relocation copy in firmware/fw_base.S allows us to load
OpenSBI firmwares anywhere in RAM and it will relocate/copy itself
to designated runtime address. This feature is very useful to U-Boot
SPL when U-Boot SPL and OpenSBI firmwares are linked at the same
runtime address (0x80000000).

To evaluate OpenSBI position independent firmwares, I would suggest
to keep it optional using compile-time option FW_PIC. This means
whenever "FW_PIC=y" is passed to OpenSBI make, everything will
be compiled as position independent and all changes for FW_PIC
will be under firmwares/ directory using "#ifdef". You will also need
to change the firmware trap handler to set up GOT on every trap
whenever FW_PIC is enabled.

Regards,
Anup