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SubjectRe: [PATCH] arm: Extend the check for RAM in /dev/mem
On Fri, Jul 12, 2019 at 02:58:18AM +0000, Raslan, KarimAllah wrote:
> On Fri, 2019-07-12 at 08:06 +0530, Anshuman Khandual wrote:
> >
> > On 07/12/2019 03:51 AM, KarimAllah Ahmed wrote:
> > >
> > > Some valid RAM can live outside kernel control (e.g. using mem= kernel
> > > command-line). For these regions, pfn_valid would return "false" causing
> > > system RAM to be mapped as uncached. Use memblock instead to identify RAM.
> >
> > Once the remaining memory is outside of the kernel (as the admin would have
> > intended with mem= command line) what is the particular concern regarding
> > the way those get mapped (cached or not) ? It is not to be used any way.
>
> They can be used by user-space which might lead to them being used by the 
> kernel. One use-case would be using them as guest memory for KVM as I detailed 
> here:
>
> https://lwn.net/Articles/778240/

From the 32-bit ARM point of view...

What if someone's already doing something similar with a non-coherent
DSP and is relying on the current behaviour? This change is a user
visible behavioural change that could end up breaking userspace.

In other words, it isn't something we should rush into.

--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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