# Documentation on the process of patching QEMU to support SFFS

Useful Links:

* bugreport: <https://bugs.libre-soc.org/show_bug.cgi?id=1132>
* (External) [qemu source code](https://github.com/qemu/qemu)
* (External) [qemu cpu flags for x86](https://www.linux-kvm.org/page/Tuning_KVM)
  
## Reasoning:

Libre-SOC builds upon the OpenPOWER ISA v3.0 but does not include
many parts of the architecture typically found on production silicon,
like AltiVec, VSX, and 128-bit hardware floating point. There are no
commercially available chips that support OpenPOWER ISA v3.0 SFFS without
VSX, AltiVec, and 128-bit hardware floating point.

Because of this, we are stuck in a chicken and egg problem where we need
the chip to test software, but we also need to test whether software
is compliant with SFFS by using such a chip. This is where QEMU as an
emulator can help us.

The goal is to add a CPU definition in QEMU that specifically emulates
an OpenPOWER ISA v3.0 SFFS chip that we can use to run tests and full
operating system images on.

## Progress and setbacks

### We have first tried using POWER7 which is available on QEMU

- POWER7 runs little endian without VSX, but is only ISA 2.06 compliant.
* The assumption was that Any additions from ISA 3.0 would be emulated
  by the Linux kernel, that however did not work.
* We tried booting [[SFFS/gentoo_bootstrap]] with a kernel compiled
  for POWER9, POWER8, and POWER7, all seem to fail with a kernel panic
  due to missing instructions.
  * The POWER9 instructions were VSX from the v3.0 ISA
  * likewise the POWER8 but also including some v2.08 instructions deprecated in v3.0
  * POWER7 included some v2.07 instructions definitely deprecated in v3.0
* We also tried booting it with a kernel built for all three POWER
versions mentioned earlier and also disabling VSX and/or AltiVec, still
fails with a kernel panic due to missing instructions.

### We have tried patching QEMU's definition for POWER9 to exclude VSX and AltiVec

- This did not work and needs considerable amounts of patching.
* The relevant files are `tcg/ppc/tcg-target.h` and
  `target/ppc/{compat.c,cpu.c,cpu.h,cpu_init.c,cpu_models.c,cpu_models.h,cpu-qom.h,cpu-param.h}`
  none seemed to work when removing such features.

### We have researched in length if the CPU capabilities feature of QEMU is available on POWER

* It is not available and needs to be added in, unlike x86 and some
  other architectures that QEMU supports.
* However, it being a feature of QEMU to start means patching it in
  will be simpler than adding a fresh feature that was not available in
  the code before.

## Conclusion as of 25 Aug 2023:

This task will need a considerable amount of additional research and work