Subject: Re: [PATCH] arm: Extend the check for RAM in /dev/mem

From: "Raslan, KarimAllah" <[email-masked]>
Date: Fri, 12 Jul 2019 08:59:59 +0000

On Fri, 2019-07-12 at 09:56 +0100, Russell King - ARM Linux admin wrote:
> 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.

Yes, that makes sense. How about adding a command-line option for this new 
behavior instead? Would this be more reasonable?

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