Re: [Xen-devel] Re: /dev/mem and /dev/kmem

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > So how about applications which simply use /dev/mem to map
> > normal domain memory? Seems this breaks compatibility
> > in user level though desired for above Xserver case... Or such
> > case is rare?
>
> This case seems not to occur in practise (no complaints so far and our
> /dev/mem has had this limitation since day one). This is perhaps not
> surprising: really not much good can come out of direct grokking of
> kernel-maintained memory without the kernel's knowledge.

IIRC the Linux /dev/mem didn't support mapping "real" memory for ages anyhow 
(causing a certain amount of confusion to users / developers) so it's not 
surprising if nobody out there is using it!  It was mainly there to keep X 
happy.

Cheers,
Mark

> > BTW, I didn't find code to protect Xen to be mapped in this path,
> > like in get_page_from_l1e(). Could you help pointing out?
>
> Not sure how you mean. Only pages that explicitly are owned by a domain and
> have a non-zero refcount can have get_page() succeed on them. So random
> Xen-owned pages are safe.
>
>  -- Keir
>
>
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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