[coyotos-dev] SMP: Dropping the low map

[Jeroen Visser]

Shap,

I re-enabled dropping of the low map in boot.S exactly as you had it before.

In order to boot up an AP I now allocate two AP bootstrap pages in low memory.

The first page contains bootstrap code, bootstrap data area and
bootstrap stack as before.

In the second page we store a copy of the cpu0_KernPDPT (or PTE
equivalent). We then manually copy IA32_PAE[3] to IA32_PAE[0] (or PTE
equivalent) to create a workable bootstrap map with a low alias.

As soon as we can we switch from the bootstrap page PDPT to the
cpu0_KernPDPT (which doesn't have the low mapping).

I think this approach will work just fine for PTE mode too. (In fact,
PTE is the only reason I allocated the second page. A mere PDPT would
fit fine inside the first page data area.)

I think the cost of needing an extra page to boot an AP is marginal
and can be ignored. The bootstrap pages after all can be unloaded when
not in use.

-JCV


On 10/22/07, Jonathan S. Shapiro <shap at eros-os.com> wrote:
> Jeroen:
>
> If it is okay with you, I am going to hold off on applying this patch. I
> want to figure out the right place to release the low memory map first.
> I have code in an old revision to do that, and I will resurrect it. I
> will then apply a cleaned-up variant of your patch. Also, I want to
> figure out the IPI issues so that we can avoid sticking BSD code in the
> tree.
>
> Unfortunately, I need to leave at the moment, so this isn't going to
> happen until tonight/tomorrow.
> --
> Jonathan S. Shapiro, Ph.D.
> Managing Director
> The EROS Group, LLC
> www.coyotos.org, www.eros-os.org
>
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