Re: [PATCH 7/8] x86: Add documentation for rd/wr fs/gs base

From: Andy Lutomirski
Date: Fri Apr 10 2015 - 16:38:46 EST

On Fri, Apr 10, 2015 at 1:22 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>> I think that your __switch_to is buggy and that's why the selectors
>> need to be zero. Is that the only issue? If so, let's fix the bug
>> instead.
> I don't think there is a bug.

So what's the issue?

If user code programs fs != 0 and fsbase != whatever is implied by fs
and the GDT/LDT, what happens? 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.


> -Andi
> --
> ak@xxxxxxxxxxxxxxx -- Speaking for myself only

Andy Lutomirski
AMA Capital Management, LLC
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