* Dan Williams <dan.j.williams(a)intel.com&gt; wrote: > On Wed, Feb 18, 2015 at 10:30 AM, Ingo Molnar <mingo(a)kernel.org&gt; wrote: > > > > * Dan Williams <dan.j.williams(a)intel.com&gt; wrote: > > > >> On Tue, Feb 17, 2015 at 12:42 AM, Boaz Harrosh <boaz(a)plexistor.com&gt; 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.