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For debugging very early boot problems we have CONFIG_PPC_EARLY_DEBUG, which allows configuring the kernel such that it unconditionally writes to a particular type of console, regardless of whether that console exists or not. This is useful sometimes when the kernel crashes before it can even determine what platform it's on, and therefore what consoles exist. However if you boot a kernel built this way on a different platform, it will generally crash because it writes to a console that doesn't exist. A particularly nasty instance of this is if you enable the hypervisor console early debug, and then boot that kernel on bare metal. The result is that the kernel calls "the hypervisor" very early in boot, but the kernel *is* the hypervisor, so we jump to the system call handler and start executing all sorts of code that isn't ready to be run. This may lead to a machine check or check stop depending on how lucky you are. Luckily there is an easy way to avoid this particular case. We simply read the MSR before installing the hooks, and if we see MSR_HV is set then we are the hypervisor and we definitely should not use the hypervisor console. Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.