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

[Dan Williams]

On Wed, Feb 18, 2015 at 10:53 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
>> On Wed, Feb 18, 2015 at 10:30 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>> >
>> > * Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> >
>> >> 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.
>> >
>> > 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?
>>
>> Yes, except that "type-12" is something picked out of the
>> air that may be invalidated by a future spec change.
>>
>> If firmware wants any driver to handle a memory range it
>> can already use E820_RESERVED.  The only reason a
>> new-type was picked in these early implementations was
>> for experiments around reserving nvdimm memory for driver
>> use, but also extending it to be covered with struct page
>> mappings.  Outside of that there is no real driving
>> reason for the new type.
>
> But ... if a user is blessed/haunted with such firmware,
> why not let new types fall back to 'reserved', which is a
> reasonable default that still allows sufficiently aware
> Linux drivers to work, right?

True.

>
>> > 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?
>>
>> I'm not sure we want firmware to repeat this confusion
>> going forward. Why support new memory types unless
>> defined by ACPI or otherwise sufficiently described by
>> E820_RESERVED?
>
> Because it would make the kernel more functional? We should
> always err on the side of allowing more functionality and
> not erect roadblocks.
>

I'm not convinced Linux is better off enabling one-off BIOS
implementations to pick non-standard numbers.  Would it not be safer
to at least confirm with the user via a configuration option, "do you
want drivers to access unknown memory types"?
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