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SubjectRe: [PATCH 7/8] x86: Add documentation for rd/wr fs/gs base
> If user code programs fs != 0 and fsbase != whatever is implied by fs
> and the GDT/LDT, what happens?

We load the value from the LDT/GDT.

> There's already a minor buglet in that
> area without wrfsbase, but fixing it would be a big performance hit
> because we don't have rdfsbase and rdgsbase to read the state
> efficiently. (Specifically, if we have gs == 0, gsbase == 0, but
> *saved* gsbase != 0, then we corrupt gsbase on context switch.)
>
> But, with the new instructions, we can do it simply, efficiently, and
> correctly in all cases. Let's do so.

We would need an instruction to write the index without changing the base.
That's not what the new instructions do.

-Andi

--
ak@linux.intel.com -- Speaking for myself only


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