Re: [PATCH] Kconfig: Make config Filter access to /dev/mem default y

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

On Thu, 15 Apr 2010 15:41:53 +0200
Michal Svoboda <michal.svoboda@agents.felk.cvut.cz> wrote:

> Jiri Kosina wrote:
> > Access to /dev/mem being filtered in PAT obviously applies only to x86. 
> > 
> > Architectures which don't do such filtering in their respective 
> > phys_mem_access_prot_allowed() still need this option.
> 
> So why not use it for all archs uniformly? Is PAT filtering better in
> some ways? 

PAT is an x86 specific hardware feature. The x86 processors can set per
page caching properties as with some other CPU designs. In the x86 case
all references to the page must have the same cache settings so the PAT
implementation has to filter /dev/mem access to avoid machine check
errors.

It's not implemented as a security feature, its a side effect of the
hardware requirements on that CPU range.

Alan