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

[Ingo Molnar]

* 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?

> > 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.

Thanks,

	Ingo
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