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Subject: Re: Readonly GDT
From: Steven Rostedt <rostedt@goodmis.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Eric Northup <digitaleric@google.com>, Kees Cook <keescook@chromium.org>,
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Date: Tue, 09 Apr 2013 20:53:54 -0400
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On Tue, 2013-04-09 at 17:43 -0700, H. Peter Anvin wrote:
> OK, thinking about the GDT here.
> 
> The GDT is quite small -- 256 bytes on i386, 128 bytes on x86-64.  As
> such, we probably don't want to allocate a full page to it for only
> that.  This means that in order to create a readonly mapping we have to
> pack GDTs from different CPUs together in the same pages, *or* we
> tolerate that other things on the same page gets reflected in the same
> mapping.

What about grouping via nodes?

> 
> However, the packing solution has the advantage of reducing address
> space consumption which matters on 32 bits: even on i386 we can easily
> burn a megabyte of address space for 4096 processors, but burning 16
> megabytes starts to hurt.

Having 4096 32 bit processors, you deserve what you get. ;-)

-- Steve

> 
> It would be important to measure the performance impact on task switch,
> though.


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