Date: Wed, 18 Feb 2015 19:30:49 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Boaz Harrosh <boaz@plexistor.com>, Matthew Wilcox <willy@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>, x86@kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "Roger C. Pao" <rcpao.enmotus@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-nvdimm <linux-nvdimm@ml01.01.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [Linux-nvdimm] [PATCH 0/2] e820: Fix handling of NvDIMM chips
Message-ID: <20150218183049.GA7032@gmail.com>
References: <54E1CF5B.9020905@plexistor.com>
 <20150216220302.GF3364@wil.cx>
 <54E2FEF2.8060701@plexistor.com>
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* Dan Williams <dan.j.williams@intel.com> wrote:

> On Tue, Feb 17, 2015 at 12:42 AM, Boaz Harrosh <boaz@plexistor.com> wrote:
> > On 02/17/2015 12:03 AM, Matthew Wilcox wrote:
> >> On Mon, Feb 16, 2015 at 01:07:07PM +0200, Boaz Harrosh wrote:
> >>> In any way this is a problem for the new type-12 NvDIMM memory chips that
> >>> are circulating around. (It is estimated that there are already 100ds of
> >>> thousands NvDIMM chips in active use)
> >>
> >> Hang on.  NV-DIMM chips don't know anyhing about E820 
> >> tables.  They don't have anything in them that says "I 
> >> am type 12!".  How they are reported is up to the 
> >> BIOS.  Just because your BIOS vendor has chosen to 
> >> report tham as type 12 doesn't mean that any other 
> >> BIOS vedor is going to have done the same thing.
> >>
> >> Fortunately, the BIOS people have all got together and 
> >> decided what they're going to do, and it's not type 
> >> 12.  Unfortunately, I think I'm bound by various 
> >> agreements to not say what they are going to do until 
> >> they do.  But putting this temporary workaround in the 
> >> kernel to accomodate one BIOS vendor's unreleased 
> >> experimental code seems like entirely the wrong idea.
> >>
> >
> > I had a feeling I'm entering an holy war ;-).
> >
> > I hope you are OK with my first patch. That an unknown 
> > type need not be reported busy, and behave same as 
> > "reserved"?
> 
> No, it seems the safe thing to do is prevent the kernel 
> from accessing any memory that it does not know the 
> side-effects of accessing.

Well, except when the kernel does know how to access it: 
when an nvdimm driver knows about its own memory region and 
knows how to handle it, right?

So is there any practical reason to mark the memory 
resource as busy in that case, instead of just adding it to 
the reserved list by default and allowing properly informed 
drivers to (exclusively) request it?

Thanks,

	Ingo
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