Re: [Linux-nvdimm] [PATCH 0/2] e820: Fix handling of NvDIMM chips

From: Dan Williams
Date: Wed Feb 18 2015 - 13:15:38 EST

On Tue, Feb 17, 2015 at 12:42 AM, Boaz Harrosh <boaz@xxxxxxxxxxxxx> 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.

> Then if we agree about PATCH-1, which is the actual fix.
> Then the 2nd patch (hence the RFC btw) is nothing more than
> a name.
> I have an old BIOS that knows nothing of NvDIMM, actually a few
> of them they all report 12.
> The fact of the matter is that all the people I've talked with,
> reported that different vendor chips, all came up type-12.
> Perhaps type-12 just means "Unknown to current BIOS"
> What is the name you suggest "type-12" "unknown-12".
> Do you understand why they all come out 12 ?

In fact it was originally "type-6" until ACPI 5 claimed that number
for official use, so these platforms, with early proof-of-concept
nvdimm support, have already gone through one transition to a new
number. They need to do the same once an official number for nvdimm
support is published.

Put another way, these early platforms are already using out-of-tree
patches for nvdimm enabling. They can continue to do so, or switch to
standard methods when the standard is published.
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