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Re: [Xen-devel] Mmap to user space (question)

To: Ruslan Nikolaev <nruslan_devel@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Mmap to user space (question)
From: Keir Fraser <keir@xxxxxxx>
Date: Wed, 06 Oct 2010 07:52:28 +0100
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Thread-topic: [Xen-devel] Mmap to user space (question)
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On 06/10/2010 00:10, "Ruslan Nikolaev" <nruslan_devel@xxxxxxxxx> wrote:

> Note: I already posted the message on xci-devel, but I think this list is
> better suited for this question. So, I duplicated it here.
> ----
> Hi
> I need to map shared_info (provided by hypervisor to each guest) to user space
> (read-only access). I am curious how I can do it, and if there are some good
> examples for the case if NO autotranslation is available. (BTW, Am I correct
> that I can use regular vm_insert_page or remap_pfn_range if autotranslation is
> available?)

I wouldn't worry about auto-translate for PV guests. It's pretty broken.

You will be using a similar mapping path to ioremap. So something like
remap_pfn_range. A function called xen_remap_domain_mfn_range sounds
promising. There's no grant table or other nonsense to worry about. You just
need to get the page mapped into a special vmarea that won't be subject to
paging or any other kernel VM machinery. Like mmap on /dev/mem and
/proc/xen/privcmd, then.

 -- Keir

> If I am correct, shared_info page is mapped initially into some fixmap area in
> the guest. However, no actual pfn exists for the page. Instead, guest gets mfn
> address from start_info.
> Now, what is the standard procedure to handle this situation? I found
> xen_remap_domain_mfn_range function, but not sure if it is what I am looking
> for. Also, I am not sure if I need to update grant tables. I would greatly
> appreciate if someone can enlighten me on this issue, and, perhaps, refer to
> some examples.
> Thank you!
> Ruslan.
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