fs/gs for thread local storage

[Vedavyas Duggirala]

Hi,

x86_64 uses fs register for tls while x86_32 uses gs register.  I am
interested in using the spare register(gs on 64bit and fs on 32bit)
for my own purpose.  Are they available ? arch_prctl has ARCH_SET_GS
option so I assume that is available. But on 32 bit the kernels struct
thread_struct  in task_struct doesn't have fs register. Does that mean
it is available or used for some other purpose.

I am curious if there is any reason why both architectures picked
different registers.

thank you,
Ved

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