Date: Thu, 10 Jul 2003 12:59:24 +0200 (CEST)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: Kirill Korotaev <dev@sw.ru>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [announce, patch] 4G/4G split on x86, 64 GB RAM (and more)
 support
In-Reply-To: <200307101450.42340.dev@sw.ru>
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On Thu, 10 Jul 2003, Kirill Korotaev wrote:

> > yes, it is also visible to user-space, otherwise user-space LDT
> > descriptors would not work. The top 16 MB of virtual memory also hosts the
> > 'virtual LDT'.
> > the top 16 MB is split up into per-CPU areas, each CPU has its own. Check
> > out atomic_kmap.h for details.

> But you need to remap it every time a switch occurs. [...]

only if the task uses an LDT - which with NPTL is very rare these days.  
But _if_ something relies on LDTs heavily then it can be done limit-less.
(we used to have nasty LDT limits due to vmalloc() limitations).

> 8. NMI is special. The only difference is that unlike usual interrupts
> NMI can interrupt returning to user-space either. This is solved easily
> by saving current cr3/esp on enter and restoring them if required on
> leave ("required"  == esp >= 0xffxxxxxx).

ok, "esp >= 0xff000000" is a good check, and it's a constant check so it
can be done very early in the entry code. The double-cr3-load is not an
issue, this is a rare race situation. I'll rework this area to be faster,
but first i wanted to concentrate on robustness. Thanks for you comments,
they are really useful!

	Ingo

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