From: sadoon <sadoon@web>
Date: Fri, 25 Aug 2023 13:47:13 +0000 (+0100)
Subject: new page
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=fdb034444ed13f0ce06b1f4dafbb3fbbaabce8e8;p=libreriscv.git

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+# 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 [Gentoo SFFS](https://libre-soc.org/SFFS/gentoo_bootstrap/) with a kernel compiled for POWER9, POWER8, and POWER7, all seem to fail with a kernel panic due to missing instructions.
+-- 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 worked.
+
+### 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 research and work