What rationale behind physical and virtual kernel memory layout ?

[telenn barz]

Hello Arun,

Thanks for answering.

On Fri, Oct 19, 2012 at 3:33 PM, Arun KS <getarunks at gmail.com> wrote:

>
> On Fri, Oct 19, 2012 at 3:03 PM, telenn barz <telenn.barz at gmail.com>wrote:
>
>>
>> Q: In the 4G/4G split case, I don't see why we have to necessarily flush
>> the TLB when switching from user-space to kernel-space ? Why the TLB
>> couldn't be shared across one user-space to kernel-space switch, and be
>> flushed only every two switches ?
>>
>
> In 4G/4G split, kernel space(KS) and user space(US) both have 4GB address
> space. So there can be a same virtual address present in KS & US which
> corresponds to two different physical addressess.
> When switching from US to KS or vice versa, the pointer to page table has
> to be changed in the MMU. So TLB which is just a cache for the page tables
> has to be flushed aswell.
>
> Yes you're right.

So considering the traditional 3G/1G split, another question is why the
kernel restricts itself to a "window" of 128MB on high memory. Can we
imagine rather that the kernel would simply allow itself to dynamically use
some virtual pages of the 3G user-space part (while changing their access
rights of course) ?

Thanks,
Telenn
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