Gdb/Armulator 源代碼分析

作者Email: Anti_chen2000@sohu.com

摘要

Gdb/Armulator 是Gdb自帶的arm7模擬器,是調(diào)試arm程序的一個(gè)好工具.而了解它的原碼結(jié)構(gòu)對(duì)擴(kuò)展它的IO功能有重要意義.本文介紹了從Armulator的啟動(dòng)到其內(nèi)部運(yùn)作和IO擴(kuò)展的大部分原代碼功能.

說(shuō)明

源代碼用的是gdb-5.0.tar+ gdb-5.0-uclinux-armulator-20021127.patch

A.和GDB間的通迅

Armulator一般和Gdb通訊有兩種方式,其一是在Gdb內(nèi)部直接調(diào)用模擬器的相關(guān)函數(shù),另一方法則是用pipe或socket傳遞RDP協(xié)議來(lái)連接Gdb和Amulator.而第一種方法是現(xiàn)在Gdb/Armulator所真正使用的(第二種是早期使用的方法),下面就分析了函數(shù)直接調(diào)用法.

函數(shù)直接調(diào)用

這個(gè)方法是由Steve (sac@cygnus.com) 修改原RDP方法而來(lái)的,Steve本人的描述如下:
/******************************************************
This directory contains the standard release of the ARMulator from
Advanced RISC Machines, and was ftp'd from.

ftp.cl.cam.ac.uk:/arm/gnu

It likes to use TCP/IP between the simulator and the host, which is
nice, but is a pain to use under anything non-unix.

I've added created a new Makefile.in (the original in Makefile.orig)
to build a version of the simulator without the TCP/IP stuff, and a
wrapper.c to link directly into gdb and the run command.

It should be possible (barring major changes in the layout of
the armulator) to upgrade the simulator by copying all the files
out of a release into this directory and renaming the Makefile.

(Except that I changed armos.c to work more simply with our
simulator rigs)
********************************************************/
/gdb/target.c,/gdb/remote_sim.c以及在/sim/arm/wrapper.c是在Armulator和Gdb的通信中起著至關(guān)重要做用的幾個(gè)文件.所有的Gdb調(diào)試命令最后都是通過(guò)在target.h里定義的target_ops結(jié)構(gòu)中的函數(shù)指針調(diào)用在/sim/arm/wrapper.c中型如sim_xxx的函數(shù)完成的.以前這些sim_xxx函數(shù)是位于/sim/common中的,是建立RDP通訊的關(guān)鍵,代碼修改后此目錄中的文件不再有用,被wrapper.c取而代之了.

以下是RDP 通訊和直接函數(shù)調(diào)用的圖示:

要清楚Armulator的執(zhí)行過(guò)程就要從它的啟動(dòng)說(shuō)起,當(dāng)你在Gdb中鍵入target sim 去激活A(yù)mulator后Gdb首先進(jìn)行命令行解釋,并將current_target指針指向sim變量,即將Armulator的調(diào)試函數(shù)集賦予Gdb,隨后的函數(shù)調(diào)用堆棧如下:

--gdbsim_open (…) in remote-sim.c.
--sim_open(…) in /sim/arm/wrapper.c/*這里Amulator對(duì)調(diào)用參數(shù)進(jìn)行分析處理*/
--*current_target->to_fetch_registers(-1) /*此函數(shù)指針實(shí)指向sim_fetch_register(…) in /sim/arm/wrapper.c*/
--sim_fetch_register(-1)/*此函數(shù)指針是在將current_target指向sim時(shí),通過(guò)注冊(cè)target_ops 結(jié)構(gòu)完成掛接的*/
sim_fetch_register (sd, rn, memory, length)
{
ARMword regval;
init ();file://就在這,Amulator進(jìn)行了初始化
…
}

至此Armulator被裝載完畢,其后Gdb就是通過(guò)target_ops(定義在target.h)結(jié)構(gòu)中的各個(gè)函數(shù)指針來(lái)完成對(duì)它的調(diào)試工.

B.Armulator 內(nèi)部機(jī)制

a.初始化

從上述可知整個(gè)模擬器的初始化入口是在wrapper.c中的init( )函數(shù),那么它到底又做了些什么呢?

(原始的Gdb5.0中的Armulator是模擬ARM7DTMI 的,而補(bǔ)丁代碼修改了memory map 并添加了timer 和uart 的IO能力使其能夠模擬AT91.因?yàn)楹笳呤菍?duì)前者的增強(qiáng),所以我們的分析以后者為準(zhǔn))

Once the armulator to reset ,the ARMul_NewState will be called.And its task is to malloc a ARMul_state stuct which saves the armulator’s states and initialize it .And the ARMul_MemoryInit() will malloc 4m ram for you.
#1static void
#2init ()
#3
#4static int done;
#5if (!done)
#6{
#7ARMul_EmulateInit ();file://Call this routine once to set up the emulator's tables.
#8state = ARMul_NewState ();
#9state->bigendSig = (big_endian ? HIGH : LOW);
#10RMul_MemoryInit (state, mem_size);file://原始代碼中的內(nèi)存初始,但現(xiàn)在無(wú)用
#11ARMul_OSInit (state);file://預(yù)裝系統(tǒng)初始化
#12ARMul_CoProInit (state);file://協(xié)處理器初始
#13state->verbose = verbosity;
#14done = 1;
#15file://the below is added for AT91
#16ARMul_SelectProcessor(state, ARM600);
#17ARMul_SetCPSR(state, USER32MODE);
#18ARMul_Reset(state);
#19}
#20}

因?yàn)檫@是補(bǔ)丁代碼,難免又冗余出現(xiàn),實(shí)際10-11行的兩處掉用是沒(méi)有實(shí)際意義的,而12行是協(xié)處理器的初始化,因?yàn)椴](méi)又模擬協(xié)處理器所以此處只是以備擴(kuò)展.

重點(diǎn)的初始化過(guò)程是在ARMul_NewState(…)中的.首先它給模擬器的核心狀態(tài)結(jié)構(gòu)ARMul_State分配了空間,這個(gè)結(jié)構(gòu)里保存了Armulator的所有方面的狀態(tài),包括arm寄存器,流水線狀態(tài)等等.

并賦予初值,我們以后就用state表示之.然后調(diào)用ARMul_Reset(…)進(jìn)行更近一步的設(shè)置.而后者又主要完成模擬器內(nèi)存結(jié)構(gòu)的分配和rom映象的加載--/sim/arm/armmem.c/mem_reset(…),IO設(shè)備的狀態(tài)初始―/sim/arm/armio.c/io_reset(…),你也可在這添加你的初始代碼.到這就完成了Armulator的裝載.

(大家注意到18行也調(diào)用了ARMul_Reset(…),這是一個(gè)BUG,使得模擬器進(jìn)行了兩次內(nèi)存分配,而浪費(fèi)了系統(tǒng)內(nèi)存.此處可刪去.)

Memory map 是所有模擬器的關(guān)鍵.Armulator由AT91向其他MCU移植時(shí)Memory map又是首先要處理的.Armulator的各個(gè)內(nèi)存區(qū)是由mem_bank_t結(jié)構(gòu)來(lái)描述的:

typedef struct mem_bank_t {
ARMword(*read_word)(ARMul_State *state, ARMword addr);
void(*write_word)(ARMul_State *state, ARMword addr, ARMword data);
unsigned longaddr, len;
char*filename;
} mem_bank_t;file://定義在armmem.h中
Armulator的整個(gè)內(nèi)存則是又此結(jié)構(gòu)的數(shù)組static mem_bank_t mem_banks[]管理的.
AT91的memory map如下:
static mem_bank_t mem_banks[] = {
/* the yuk's below are to work around a uClinux/mount options problem */
{ real_read_word,real_write_word,0x01000000, 0x00400000, }, /* 2.4 */
{ real_read_word,_write_word,0x01400000, 0x00400000, "boot.rom"},
{ real_read_word,real_write_word,0x02000000, 0x00400000, }, /* 2.0 */
{ real_read_word,real_write_word,0x02400000, 0x00001000, }, /* yuk!*/
{ real_read_word,_write_word,0x04000000, 0x00400000, "boot.rom"},
{ real_read_word,real_write_word,0x00000000, 0x00004000, },
{ io_read_word,io_write_word,0xf0000000, 0x10000000, },
{ fail_read_word,fail_write_word, 0, 0 }
};

根據(jù)mem_banks,mem_reset( )將分配空間,加載boot.rom文件.

(原來(lái)的內(nèi)存是由ARMul_MemoryExit( )釋放的,但補(bǔ)丁后的代碼就沒(méi)了釋放功能,這也是需要糾正的地方)

a.指令流

Armulator 加載完成后,就開(kāi)始等待Gdb的運(yùn)行命令了.最終/sim/wrapper.c/sim_resume( )是啟動(dòng)arm指令執(zhí)行的地方.

Sim_resume( )根據(jù)Gdb的要求選擇用/sim/arm/arminit.c/ARMul_DoInstr()還是用/sim/arm/arminit.c/ARMul_DoProg()來(lái)調(diào)用 流水線模擬函數(shù)/sim/arm/armemu.c/ARMul_Emulate32().ARMul_DoInstr()和ARMul_DoProg()的區(qū)別就是一個(gè)單步執(zhí)行,一個(gè)連續(xù)執(zhí)行指令. ARMul_DoProg()又不停的判斷state->Emulate是否為STOP,如果是,模擬器又將停下等待Gdb的調(diào)試.

而在arm/armemu.c, /arm/armvirt.c 和 /arm/armsupp.c中的函數(shù)則模擬指令預(yù)取,指令譯碼,指令執(zhí)行以及數(shù)據(jù)回寫(xiě)的功能.這三個(gè)文件時(shí)可以說(shuō)時(shí)Armulator的核心!

b.中斷

Armulator 的中斷機(jī)制主要靠以下兩個(gè)例程實(shí)現(xiàn):

1.IntPending(): 用來(lái)檢測(cè)state中的各個(gè)中斷標(biāo)志是否置位,從而判斷是否又需要中斷.

2.ARMul_Abort():當(dāng)需要中斷時(shí),用來(lái)改變處理器模式,并將pc指向相應(yīng)的中斷向量.

在流水線函數(shù)ARMul_Emulate32()執(zhí)行當(dāng)中,有多處調(diào)用IntPending() 去檢測(cè)中斷.而Ispending() 也十分簡(jiǎn)單,它僅僅判斷state中的四個(gè)變量:

State->Exception : 中斷使能標(biāo)志.
State->NresetSig : reset 中斷信號(hào).
State->NirqSig : irq 中斷信號(hào).
State->NfiqSig : fiq 中斷信號(hào).
所以當(dāng)你的虛擬外設(shè)產(chǎn)生中斷時(shí),你只要調(diào)用/sim/arm/armio.c/ update_int()即可:
static void update_int(ARMul_State *state)
{
ARMwordrequests = state->io.intsr state->io.intmr;

state->NfiqSig = (requests 0x000f) ? LOW : HIGH;
state->NirqSig = (requests 0xfff0) ? LOW : HIGH;
}

b.讀寫(xiě)操作

無(wú)論是CPU指令還是Gdb調(diào)試時(shí)讀寫(xiě)內(nèi)存或IO空間,最后都將要落到/armvirt.c/getword(), /armvirt.c/putword()這兩個(gè)函數(shù)身上.

在原始代碼中這兩個(gè)函數(shù)馬上就進(jìn)行內(nèi)存數(shù)組的讀寫(xiě)了.而補(bǔ)丁代碼的流程如下:

--getword()/putword()
--mmu_read_data()/mmu_write_data() in armmmu.c
/*進(jìn)行mmu的地址轉(zhuǎn)換和cache 查詢*/
--real_read_data()/real_write_data() :讀寫(xiě)ram,rom
io_read_data()/io_write_data() :讀寫(xiě)IO空間
_write_data()/fail_read_data()/fail_write_data() :非法讀寫(xiě),如地址錯(cuò)誤,rom讀操作等.

可以看出最后幾個(gè)函數(shù)的選擇是由讀寫(xiě)地址在相應(yīng)的mem_bank_t結(jié)構(gòu)中的讀寫(xiě)函數(shù)指針決定的.

c.設(shè)備同步

寫(xiě)這篇文章的初衷是讓讀者能很快進(jìn)入Armulator的移植和IO擴(kuò)展的實(shí)際工作中去.所以這里有必要討論一下IO設(shè)備和CPU的同步問(wèn)題.很顯然我們模擬的設(shè)備不能太快,也不能太慢.快了CPU正常的指令流將被堵塞,慢了就無(wú)法反映操作系統(tǒng)的實(shí)時(shí)性.也就是說(shuō)設(shè)備的速度和指令流要有個(gè)比例關(guān)系,即要有一定的同步.

Armulator 中有個(gè)很好的接口:

ARMul_ScheduleEvent (ARMul_State * state, unsigned long delay, unsigned (*what) ()) in armvirt.c

它的目的就是注冊(cè)你的同步例程,并且每個(gè)時(shí)鐘周期即ARMul_Emulate32()將調(diào)用ARMul_ScheduleEvent()查看是否需要同步你的設(shè)備.

也許你在ARMul_Emulate32()中還發(fā)現(xiàn)了/sim/arm/armio.c/io_do_cycle(),沒(méi)錯(cuò)它是AT91的timer和uart用來(lái)和指令流同步的函數(shù),但我并不贊成你象這樣把自己的同步例程直接放入指令執(zhí)行過(guò)程中,破壞代碼的結(jié)構(gòu)性.

a. 源文件描述