Re: L4 Pistachio: getting started guides / tutorials

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Am 29.11.2011 21:11, schrieb Semion Prihodko:
> Hi. Can someone explain me what Sigma0 server is for? As far as I know
> it serves as a pager of root task and when a page fault occurs, it maps=

> the page to the physical page with a same address. Am I right? If so,
> why should root task request pages if it can simply access them
> directly?=20

You are right about sigma0, but the roottask cannot access physical
memory directly. Only the kernel can do that. Sigma0 makes sure that the
same physical page is never accessed by more than one process (unless it
is explicitly mapped between processes).

> Furthermore, I need to implement my custom pager for
> non-privileged tasks (sigma0 is not accessible for them, I hope), or th=
e
> root task itself (for example, if I need a persistent virtual memory).
> So why should I need Sigma0? Does it mean I can implement my own Sigma0=
?

I suppose any thread can send a request to sigma0 and get a mapping
back. However, sigma0 won't handle pagefaults for your tasks if you
don't set sigma0 as their pager.
As for your own pager: You could replace sigma0 with something of your
own, or extend our sigma0 with the functionality you need. You could
also implement your own pager, have it request all available memory from
sigma0 (I have one application here which does exactly that) and then
set it as pager for all newly created tasks. The roottask always starts
with sigma0 set as its pager, but it should be able to change its own
pager to a different thread using the ThreadControl system call.

Hope this helps

Jens


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