Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

[Andy Lutomirski]

On Fri, Apr 10, 2015 at 1:41 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>> It wouldn't take any additional memory at all.  Currently we have 8k
>> of "debug" stack which is really two 4k pieces, and you're putting the
>> kernel gs base in the bottom word.  I'm suggesting that you duplicate
>> the kernel gs base at the bottom work and the bottom word + 4k.  We
>> already have a hard limit of 4k of debug stack because of the IST
>> shift mechanism -- it really is two separate 4k stacks, not one 8k
>> stack.
>
> Seems like a hack. What happens if we add an uneven number of stacks?

This works for DEBUG_STKSZ == n * EXCEPTION_STKSZ for any n.  Just
duplicate the pointer n times.

I think all of this stems from unfortunate naming.  DEBUG_STACK isn't
one stack -- it's a debug stack *array*.  The IST shift mechanism
means that we can use different entries in that array as our stacks
depending on how deeply nested we are.

>
> Just handling it in the code is simple enough.

It seems to account for over half the asm diff.  I'm talking about the
addition of approximately two lines of C and the removal of a huge
chunk of the asm diff.

--Andy
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