Hi, This might be considered a minor issue, but I submit my findings anyhow.
In some distros, the msr module is not loaded by default. (I guess not built in by default, would be more correct.) I often forget to load it before using "rdmsr" or "wrmsr". It was never an issue before, but as of Kernel 4.4-rc1 it causes a segmentation fault crash. Example: # rdmsr -a 0x19a Segmentation fault (core dumped) It could be argued that the msr-tools commands should check if the module is loaded and abort with some error message if not (or, and as turbostat now does, just load the module). However, what I do not know is if there might be other effected use cases. The kernel was bisected, and this is the result: 9a2bc335f100a0f6ee6392b9f97ac4188d84db1d is the first bad commit commit 9a2bc335f100a0f6ee6392b9f97ac4188d84db1d Author: Borislav Petkov <[email protected]> Date: Tue Oct 20 11:54:44 2015 +0200 x86/microcode: Unmodularize the microcode driver Make CONFIG_MICROCODE a bool. It was practically a bool already anyway, since early loader was forcing it to =y. ... Since I sometimes mess up using git bisect, and end up at some random result, the above was double checked manually: 9a2bc335f100a0f6ee6392b9f97ac4188d84db1d has the issue 81ffdcdd97d94110627caa81c23d5d780083731d (the previous commit) does not have the issue. As a potentially interesting side note, the segmentation fault does not occur if I use sudo instead. Example: $ sudo rdmsr -a 0x19a $ Of course, if I load the module then things work as expected: # modprobe msr # rdmsr -a 0x19a 0 0 0 0 0 0 0 0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
