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 10:53 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"?

Well, we could emit a warning (or taint the kernel), to 
keep the user informed that there's a version mismatch 
between kernel and firmware - but otherwise still allow 
informed drivers to register that region?

Thanks,

	Ingo
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