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

From: "Raslan, KarimAllah" <karahmed@amazon•de>
To: "linux@armlinux•org.uk" <linux@armlinux•org.uk>
Cc: "mark.rutland@arm•com" <mark.rutland@arm•com>,
	"anders.roxell@linaro•org" <anders.roxell@linaro•org>,
	"yuzhao@google•com" <yuzhao@google•com>,
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	"ard.biesheuvel@linaro•org" <ard.biesheuvel@linaro•org>,
	"info@metux•net" <info@metux•net>,
	"anshuman.khandual@arm•com" <anshuman.khandual@arm•com>,
	"linux-kernel@vger•kernel.org" <linux-kernel@vger•kernel.org>,
	"rppt@linux•ibm.com" <rppt@linux•ibm.com>,
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	"james.morse@arm•com" <james.morse@arm•com>,
	"catalin.marinas@arm•com" <catalin.marinas@arm•com>,
	"tglx@linutronix•de" <tglx@linutronix•de>,
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	"akpm@linux-foundation•org" <akpm@linux-foundation•org>,
	"linux-arm-kernel@lists•infradead.org"
	<linux-arm-kernel@lists•infradead.org>
Subject: Re: [PATCH] arm: Extend the check for RAM in /dev/mem
Date: Fri, 12 Jul 2019 08:59:59 +0000	[thread overview]
Message-ID: <1562921998.1345.15.camel@amazon.de> (raw)
In-Reply-To: <20190712085602.v2tncu5tsngtvbww@shell.armlinux.org.uk>

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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