Messages in this thread | | | Date | Thu, 04 Sep 2008 09:49:42 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [x86] fs, gs purpose & multicore prog |
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Eric Lacombe wrote: > Hello, > > I've some questions about IA-32e in Linux. > > - What is the FS and GS segments role inside the kernel ? > (I was thinking about thread local storage) >
In a 32-bit kernel %fs is the base of the per-cpu data area. In a 64-bit kernel %gs points to the pda (processor data area). The pda is a single structure, whereas per-cpu data is a section that per-cpu variables get put into.
> - When I do a "mov %fs ..." instruction (in a module), it seems that %fs is > equal to 0 (idem for %gs). Are these registers not always filled ? >
On 32-bit they will always have a value, or you'll get a GPF. On 64-bit the value of the selector doesn't matter because the MSRs are the real content.
> - What is the purpose of MSR_FS_BASE and MSR_GS_BASE ? > (I thought they were filled with "gdt[fs_entry].base") > On 64-bit, the GDT isn't large enough to hold a 64-bit offset, so it only stores the low 32-bits. When you load a segment register with a selector, it picks up from the gdt. If you want a full 64-bit offset, you need to write it to the msr.
> - My last question is about the kernel programation of multi-core (or > multiprocessor) > architecture. I don't see a lot of documentation about that on Internet. > Do you have some docs/urls about this topic. > Maybe someone can briefly explain how the execution flow are given to the > different cores. >
To the kernel they're all just cpus, and it runs tasks on them as usual. There are a few tweaks in the scheduler to pay attention to the shared caches and so on.
J
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