Porting uclinux to MIPS

Transcription

1 Porting uclinux to MIPS This document describes how to porting uclinux to MIPS platform. And also describes how to porting GNU toolchain and uclibc on that platform. The last version of this document can always be found at 1.About this document 1.1 What this document tells and doesn t tell 1.2 Copyright notice 1.3 The last version of this document 2. General information of uclinux 2.1 Introduction of uclinux 2.2 How about the GNU toolchain and c lib 3. MIPS 3.1 Introduction of MIPS 3.2 MIPS without MMU 4. Porting uclinux 4.1 uclinux kernel structure 4.2 Kernel mode and user mode check 4.3 Memory accessing check 4.4 Fork and Vfork 4.5 binfmt_flat.c 4.6 Reserve instruction and linux 4.7 Memory page fault handle 5. Porting GNU toolchain 5.1 GNU toolchain and uclinux 5.2 Porting ELF2FLT 5.3 Reserve instructions and toolchain 5.4 Soft floating 6. Porting uclibc 6.1 Introduction of uclibc 6.2 Stack align to 8 bytes 6.3 Pthread 6.4 float-point instruction 7. JIT Simulator 7.1 Introduction 7.2 How to use it? 7.3 Where can I get it?

2 8. Other issue 8.1 Share lib on uclinux 8.2 PIC or NO-PIC? 9. Get and build packages 1.About this document 1.1 What this document tells and doesn t tell This document descript how to porting uclinux, uclibc and GNU toolchain to MIPS platform. And it tells people where to get the patches and tools. We didn t list every place we modify in the original code, because reader can see all the changes in our patch file. Also this document doesn t give details about MIPS and uclinux. We assume the readers have already known about these. We will use the special version of hardware MIPSR3000 as samples. Hope this document helps. 1.2 Copyright notice Copyright (c) Qiu Liming (qiulm@xiptech.com ) Chang Xinlin (zxl@xiptech.com) Xipos Tech. ( ) Personal user can get and read this document freely. Please don t use it in business without the permit. 1.3 The last version of this document 2. General information of uclinux 2.1 Introduction of uclinux uclinux is the most popular OS used in no MMU device. Though uclinux works the same with linux in many ways, they have some differences, such as fork, mmap, stack size, memory fragment. We should pay attention to these problems when we porting AP from linux to uclinux. However, uclinux is still one of the best choice in no MMU system.

3 The newest version of uclinux kernel can be obtained at The official release supports two platform, arm and m68k. uclinux has been ported to many other platforms. As we know, MIPS version of uclinux 2.4.1x is done by some companies. But there is no patch is published. We do the porting job and now release it to the public. 2.2 How about the GNU toolchain and uclibc uclibc is the standard C lib for uclinux. It supports MIPS already. But to MIPSR3000, there are some place had to be changed. GNU toolchain can be used to build linux or uclinux, no matter kernel and AP. But a tool named elf2flt, which is used to generate the flat format file for uclinux, need to be added to toolchain. The original version of elf2flt doesn t support MIPS. 3. MIPS 3.1 Introduction of MIPS We can get all information about MIPS at Here we only list some features of MIPS, TLB can be seen by software Delay slot Access unaligned address will cause exception Div zero will NOT cause exception 3.2 MIPS without MMU Figure 1

4 Figure 1 is the memory mapping between virtual address and physical address of no MMU version MIPS. Only kuseg can be accessed by user mode program. When the physical RAM is located at 0x , some thing special will happen. No user mode program can run because they cannot access any RAM! Unfortunately many devices arrange their RAM to 0x We must solve this problem. Later section you will see how. 4. Porting uclinux 4.1 uclinux kernel structure This section descript the structure of uclinux kernel. Uclinux kernel can be generated as following, 1. Get linux kernel source code 2. Get the corresponding uclinux patch 3. Patch it to linux source code tree, then we get uclinux kernel There are 5 directories are added into the linux tree mmnommu arch/armnommu arch/m68knommu include/asm-armnommu include/asm-m68knommu For supporting flat file format, fs/binfmt_flat.c is also added. To start our tour of porting uclinux to MIPS, let s copy include/asm -mips to include/asm-mips, and then copy arch/mips to arch/mipsnommu. After these steps, change the Makefile to disable arch/mips and enable arch/mipsnommu. 4.2 Kernel mode and user mode In uclinux/linux, OS kernel run in kernel mode, AP and lib run in user mode. But as we discuss in section 3.2, user mode program cannot be executed on some platforms. What can we do to let uclinux run on them? Our solution is maintain MIPS CPU running at kernel mode, and cheats the uclinux kernel when it tries to do the switch between user mode and kernel mode. The first step is change start_thread (include/asm-mipsnommu/processor.h) #define start_thread(regs, new_pc, new_sp) do { \ /* New thread loses kernel and FPU privileges. */ \ regs->cp0_status = (regs->cp0_status & ~(ST0_CU0 ST0_KSU ST0_CU1));\ regs->cp0_status = (regs->cp0_status & ~(ST0_CU0 ST0_KSU ST0_CU1)) KU_USER;\ regs->cp0_epc = new_pc; \ regs->regs[29] = new_sp; \ current->thread.current_ds = USER_DS; \ } while (0)

5 This macro will be executed when kernel creates user mode program. It will set KU_USER bit to force MIPS CPU into user mode. Once we remoce this bit, MIPS CPU won t go into user mode anymore. The second step is to cheat uclinux. When exception happens, uclinux check if it is running at user mode. If the answer is yes, the stack needs to be switched to kernel mode stack. After exception is handled, uclinux check this again to decide if the stack needs to be switched back. MIPS linux uses two methods to do the check, bit CU0 of CP0_STATUS and bit KU_USER of CP0_STATUS. We cannot use KU_USER in our uclinux. Then we use CU0 for all checks. This step will change the following files, arch/mipsnommu/kernel/entry.s arch/mipsnommu/kernel/scall_o32.s include/arch/mipnommu/stackframe.h include/arch/mipsnommu/ptrace.h At the last of this section, we will discuss about scall_o32.s. This file handles system call. When system call is done, the CPU should return to the place where the call is made. But if the call is come from user mode, signal and schedule will be handled first. Code in scall_o32.s works fine when the call is really come from user mode. The problem is there is no check here, if the call is come from kernel mode, will this code work fine too? 4.3 Memory accessing check To enhance the security of linux, kernel will check if a pointer is legal before using it. Because it s hard to distinguish kernel mode address and user mode address in MIPS uclinux., the memory check should be removed. At first we need to change access_ok (include/asm-mipsnommu/uaccess.h), most memory check is accomplished by it. Then we need to modify some functions written in assembly, such as strncpy_from_user_asm. These functions will do the pointer check by itself. 4.4 Fork and vfork Uclinux doesn t support fork, it only supports vfork. To avoid user mode program call fork, we should change sys_fork in MIPS uclinux. We just need to change the parameters sys_fork pass to do_fork, let sys_fork work as vfork. 4.5 binfmt_flat.c

6 This file handles the flat format for uclinux. It only supports arm and m68k. We will make it support MIPS as well. Details is discuss in Reserve instruction and linux MIPS I has some instructions to handle unaligned address access. (lwl/lwr,swl/swr) But they are not implemented in every MIPS CPU product. Such CPU will cause a Reserve Instruction exception when executing them. The normal way to solve this is handle them in do_ri. But this is not the best way because it slows down the system. In function memcpy will use these instructions, imaging what will happen when you do memory copy. We must remove these legal but not implemented instructions from our MIPS uclinux. Gcc will emit these instructions too. We had better to modify Gcc to avoid this problem. 4.7 Memory page fault handle When page fault happens, linux will try to find out the corresponding VMA. And then try to fix up the fault. This is not be necessary in uclinux. Therefore the code handle page fault can be removed. 5. Porting GNU toolchain 5.1 GNU toolchain and uclinux As we discuss before, GNU toolchain can be used to develop MIPS uclinux kernel and AP. The only thing we need to do is porting elf2flt. Thanks for the great job done by Erik Andersen (andersen@uclibc.org), we can donwload, config, compile and install GNU toolchain easily. And his job is the base of ours. 5.2 Porting ELF2FLT GNU toolchain doesn t support flat format. Ld will generate ELF format file, and then use elf2flt to convert it to flat file. Now we introduce the flat format in brief. A flat file has 5 parts, they are file header, text section, data section, bss section and relocate table. The file header includes the sizes and offsets of other parts, also includes the entry point and characteristics of the file. The relocate table includes a got table and some other relocate items. The got table is located at the beginning of data section, end ends with a dword 1. After install elf2flt, a script file ld-elf2flt takes the place of the original ld, the old ld is renamed to mipsel-uclibc-ld.real. To create a flat file, we need to do like this, mipsel-uclibc-gcc Wl,-elf2flt foo.c o foo

7 Gcc will call ld (in fact it calls ld-elf2flt) to link after compile foo.c. And ld-elf2flt will call the old ld twice to create two ELF files, foo.elf and foo.gdb. foo.elf is an object file, includes reloc section. foo.gdb is executable file, includes got table. ld-elf2flt then call elf2flt, uses foo.elf and foo.gdb as input. Elf2flt uses information from these two ELF file to create relocate table. We must notice that the memory map of two ELF file is not the same, because of the existing of got table. Elf2flt will calculate the got table size. The original version of elf2flt assumes every data section in object file align at 4 bytes. This condition can not be guaranteed in MIPS object file. We have fixed it. Elf2flt uses elf2flt.ld as ld script. Base address of the whole module is set to 0 in elf2flt.ld. It cause got table contains some zero value item. binfmt_flat.c won t handle these items because it need to support c++ programs. We change the base address to 0x40000 and solve this problem. 5.3 Reserve instructions and toolchain As we descript in 4.6, sometimes GNU toolchain need to be modified to avoid emitting some instructions. How to do this is a large issue, we won t talk about it in this document. The paper GNU Compiler Collection (GCC) Internals will be a good reference for doing this job. If you only want to remove unaligned memory access instructions, you can change tc-mips.c in GAS. But you had best to change gcc to generate more efficient code. 5.4 Soft floating gcc supports soft-floating. You only need to add msoft-float to your Makefile. 6. Porting uclibc 6.1 Introduction of uclibc uclibc can work on MIPS platform without many porting job. Though we still have some thing to do. 6.2 Stack align to 8 bytes The MIPS version gcc has a macro va_arg, just like other versions. The stack pointer need to be aligned at 8 bytes when use this macro in MIPS. It will cause error when the macro handles 8 bytes long parameter otherwise. We make the stack pointer align to 8 bytes in uclibc startup code. 6.3 Pthread Pthread code in uclibc use MIPS II instruction in function testandset. If the CPU only supports MIPS I, we should remove them. We use sysmips(mips_atomic_set ) instead of the MIPS

8 II instructions. 6.4 float-point instruction MIPS version uclibc will save and restore floating-point registers in setjmp and longjmp. We must remove the code if the CPU has no floating-point processor. (see libc/linux/mips/ longjmp.c and libc/linux/mips/setjmp_aux.c ) 7. JIT Simulator 7.1 Introduction The best way to be a expert of porting uclinux to MIPS is to do all porting job by yourself. But every one cannot get a MIPS hardware. Even you have one, it s wasting tim e to download and write the code to flash rom. We would like to introduce our Xiptech Simulator for MIPS to you. It is an instruction set system level simulator. It emulates a MIPSR3000 CPU and some virtual devices. It can run MIPS binary code directly, including linux/uclinux kernel, libs and applications. The main feature of our simulator is its world leading performance. It uses a just in time compile engine and can run faster than 50 mips on a P3 650 notebook. We offer a demo version to personal users. This version doesn t contain some advance functions, but it s enough to learn how to porting linux/uclinux/libs/gui/application/gdb to MIPS platform. Please don t use it in business without permit. 7.2 How to use it? The demo version simulator runs on MS Windows 98/Me/2000/XP. If you need linux version please contact us. A user guide document is included in the package. In fact, using simulator just like use the hardware. You need to build an OS kernel image and give it to the simulator. Then simulator will boot and run your kernel. Our patch file for MIPS uclinux contains all drivers for simulator. You can find them in arch/mipnommu/mach-xipos. 7.3 Where can I get it? 8. Other issue 8.1 Share lib on uclinux Modern OS provide share lib or DLL. Some DLL systems don t support process independent data section, such as MS Win3.1. Some others do, such as linux. In no MMU systems, it s not easy to

9 provide the second type of DLL. Therefore normal linux share libs cannot be used in uclinux. As we know, some companies have expanded uclinux to support share lib under arm and m68k platform. We do the same job on MIPS platform. But the code is under testing, we will release it in next version. 8.2 PIC or NO-PIC? Linux uses PIC code in its share lib and applications. In uclinux we have choices, PIC or NO-PIC? Based on our testing, NO-PIC code run faster than PIC code. (About 30%) Using NO-PIC will improve the system s performance. But NO -PIC code will cause two problems, much bigger executable file size and more time to load the program. You can make decision based on your situation. By the way, to use NO-PIC code, some reloc types, such as REL_HI16, should be handled in elf2flt. 9. Get and build packages Build toolchain At first you MUST connect to Internet when you build the toolchain. Download toolchain.tar.gz from then unzip it and make. All the source code of toolchain will be download and config automatically. The toolchain will be installed to /opt/toolcahin by default. Before you use it, You need to export PATH=/opt/toolchain/bin:$PATH If you want to change the install path, you need to change TARGET_PATH in Makefile before you make. Maybe you need to modify uclibc for your platform, please do it after the make process. Do your change and then make uclibc This operation will rebuild and install the uclibc. toolchain.tar.gz comes from Erik Andersen. We just do a little change and use our elf2flt for MIPS. More information and the newest version of this package can be get on Build uclinux Dowdload the following files,

10 Unzip and patch, mkdir mips-uclinux cd mips-uclinux bunzip2 <../linux tar.gz2 tar xvf - gunzip../uclinux uc1.diff.gz gunzip../uclinux uc1-mips.diff.gz tar zxvf../romfs.tar.gz cd linux patch -p1 <../uclinux uc1.diff patch -p1 <../uclinux uc1-mips.diff Build kernel make mrproper cp xipos.config.config make menuconfig make dep make The file uclinux.bin will be created, and then use our Xiptech simulator for MIPS to run it.