Re: 4k stacks in 2.6

[Brian Gerst <bgerst@didntduck.org>]

Guy Sotomayor wrote:

> On Thu, 2004-05-27 at 07:55, Linus Torvalds wrote:
> 
> 
>>"minor implementation detail"?
>>
>>You need to get to the thread info _some_ way, and you need to get to it
>>_fast_. There are really no sane alternatives. I certainly do not want to
>>play games with segments.
> 
> 
> While segments on x86 are in general to be avoided (aka the 286
> segmented memory models) they can be useful for some things in the
> kernel.
> 
> Here's a couple of examples:
>       * dereference gs:0 to get the thread info.  The first element in
>         the structure is its linear address (ie usable for being deref'd
>         off of DS).

The only problem with using %gs as a base register is that reloading it 
on every entry and exit is rather expensive (GDT access and priviledge 
checks) compared to masking bits off %esp.  x86-64 can get away with it 
because it has the swapgs instruction which makes it efficient to use.

>       * use SS to enforce the stack limit.  This way you'd absolutely
>         get an exception when there was a stack overflow (underflow). 
>         SS gets reloaded on entry into the kernel and on interrupts
>         anyway so there really shouldn't be a performance impact.  I
>         haven't looked at all the (potential) gcc implications here so
>         this one may not be completely doable.

Not possible.  GCC completely assumes that we are working with a single 
flat address space.  It has no concept of segmentation at all.

--
				Brian Gerst