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tr_trs:kbp [2018/08/03 10:49]
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tr_trs:kbp [2018/08/03 10:49] (текущий)
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 {{:fs_research:kernels:linux:kernelinitparttwo.png|}} {{:fs_research:kernels:linux:kernelinitparttwo.png|}}
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 Architecture-independent Linux Kernel Initialization Architecture-independent Linux Kernel Initialization
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 But here’s the kicker for us. This idle loop is the end of the long thread we followed since boot, it’s the final descendent of the very first jump executed by the processor after power up. All of this mess, from reset vector to BIOS to MBR to boot loader to real-mode kernel to protected-mode kernel, all of it leads right here, jump by jump by jump it ends in the idle loop for the boot processor, cpu_idle(). Which is really kind of cool. However, this can’t be the whole story otherwise the computer would do no work. But here’s the kicker for us. This idle loop is the end of the long thread we followed since boot, it’s the final descendent of the very first jump executed by the processor after power up. All of this mess, from reset vector to BIOS to MBR to boot loader to real-mode kernel to protected-mode kernel, all of it leads right here, jump by jump by jump it ends in the idle loop for the boot processor, cpu_idle(). Which is really kind of cool. However, this can’t be the whole story otherwise the computer would do no work.
  
-At this point, the kernel thread started previously is ready to kick in, displacing process 0 and its idle thread. And so it does, at which point kernel_init() starts running since it was given as the thread entry point. kernel_init() is responsible for initializing the remaining CPUs in the system, which have been halted since boot. All of the code we’ve seen so far has been executed in a single CPU, called the boot processor. As the other CPUs, called application processors, are started they come up in real-mode and must run through several initializations as well. Many of the code paths are common, as you can see in the code for startup_32, but there are slight forks taken by the late-coming application processors.+At this point, the kernel thread started previously is ready to kick in, displacing process 0 and its idle thread. And so it does, at which point kernel_init() starts running since it was given as the thread entry point. kernel_init() is responsible for initializing the remaining CPUs in the system, which have been halted since boot. All of the code we’ve seen so far has been executed in a single CPU, called the boot processor. As the other CPUs, called application processors, are started they come up in real-mode and must run through several initializations as well. Many of the code paths are common, as you can see in the code for startup_32, but there are slight forks taken by the late-coming application processors. Finally, kernel_init() calls init_post(), which tries to execute a user-mode process in the following order: /sbin/init, /etc/init, /bin/init, and /bin/sh. If all fail, the kernel will panic. Luckily init is usually there, and starts running as PID 1. It checks its configuration file to figure out which processes to launch, which might include X11 Windows, programs for logging in on the console, network daemons, and so on. Thus ends the boot process as yet another Linux box starts running somewhere. May your uptime be long and untroubled. 
 + 
 +The process for Windows is similar in many ways, given the common architecture. Many of the same problems are faced and similar initializations must be done. When it comes to boot one of the biggest differences is that Windows packs all of the real-mode kernel code, and some of the initial protected mode code, into the boot loader itself (C:\NTLDR). So instead of having two regions in the same kernel image, Windows uses different binary images. Plus Linux completely separates boot loader and kernel; in a way this automatically falls out of the open source process. The diagram below shows the main bits for the Windows kernel: 
 + 
 +{{:fs_research:kernels:linux:windowskernelinit.png|}} 
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 +Windows Kernel Initialization 
 + 
 +The Windows user-mode start-up is naturally very different. There’s no /sbin/init, but rather Csrss.exe and Winlogon.exe. Winlogon spawns Services.exe, which starts all of the Windows Services, and Lsass.exe, the local security authentication subsystem. The classic Windows login dialog runs in the context of Winlogon. 
 + 
 +This is the end of this boot series. Thanks everyone for reading and for feedback. I’m sorry some things got superficial treatment; I’ve gotta start somewhere and only so much fits into blog-sized bites. But nothing like a day after the next; my plan is to do regular “Software Illustrated” posts like this series along with other topics. Meanwhile, here are some resources: 
 + 
 +    * The best, most important resource, is source code for real kernels, either Linux or one of the BSDs. 
 +    * Intel publishes excellent Software Developer’s Manuals, which you can download for free. 
 +    * Understanding the Linux Kernel is a good book and walks through a lot of the Linux Kernel sources. It’s getting outdated and it’s dry, but I’d still recommend it to anyone who wants to grok the kernel. Linux Device Drivers is more fun, teaches well, but is limited in scope. Finally, Patrick Moroney suggested Linux Kernel Development by Robert Love in the comments for this post. I’ve heard other positive reviews for that book, so it sounds worth checking out. 
 +    * For Windows, the best reference by far is Windows Internals by David Solomon and Mark Russinovich, the latter of Sysinternals fame. This is a great book, well-written and thorough. The main downside is the lack of source code. 
 + 
 +[Update: In a comment below, Nix covered a lot of ground on the initial root file system that I glossed over. Thanks to Marius Barbu for catching a mistake where I wrote "CR3" instead of GDTR]
  
-Finallykernel_init() calls init_post(), which tries to execute a user-mode process in the following order/sbin/init, /etc/init, /bin/init, and /bin/sh. If all fail, the kernel will panic. Luckily init is usually there, and starts running as PID 1. +June 232008 at 2:20 am
tr_trs/kbp.txt · Последние изменения: 2018/08/03 10:49 — dwadmin