a basic question

a basic question

[Nitin Gupta]

Hi,
    These are the questions that have been troubling me for long time.
I'm beginning to work on a vmm project, so kindly spare a min to
answer these:

- How does processor know that 3GB-4GB is mapped linearly on first 1GB
of memory. Is there a pagetable for this segment mapping it linearly?

- Why isn't it like this  - userspace tasks have 4GB virtual address
space and for kernel also a 4GB virtual address space that is linearly
mapped to fist 4GB of memory.


Thanks

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Re: a basic question

[Rik van Riel]

On Sat, 22 Oct 2005, Nitin Gupta wrote:

> - How does processor know that 3GB-4GB is mapped linearly on first 1GB
> of memory. Is there a pagetable for this segment mapping it linearly?

Yes, there are page tables for this.

> - Why isn't it like this  - userspace tasks have 4GB virtual address
> space and for kernel also a 4GB virtual address space that is linearly
> mapped to fist 4GB of memory.

With the 4:4 split patch, this is done.  However, there is a
cost to this approach - every time the system switches from
user mode to kernel mode it goes through a context switch.

An extra two context swiches on every system call, every 
interrupt. For most systems the gained space is simply not 
worth the time overhead.

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