linux-kernel - Re: [Linux-nvdimm] another pmem variant

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Message-ID: <CAPcyv4jYMziV3F5FE8S5jT6c_xtMJ0R45x3huKJ_JKOi2WPy4w@mail.gmail.com>
Date:	Wed, 25 Mar 2015 10:00:26 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	linux-nvdimm <linux-nvdimm@...1.01.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [Linux-nvdimm] another pmem variant

On Wed, Mar 25, 2015 at 9:44 AM, Christoph Hellwig <hch@....de> wrote:
> On Wed, Mar 25, 2015 at 09:33:52AM -0700, Dan Williams wrote:
>> This is mostly ok and does not collide too much with the upcoming ACPI
>> mechanism for this stuff.  I do worry that the new
>> "memmap=nn[KMG]!ss[KMG]" kernel command line option will only be
>> relevant for at most one kernel cycle given the imminent publication
>> of the spec that unblocks our release.
>
> I don't think we can just get rid of it as legacy systems won't be
> upgraded to the new discovery mechanism.  Or do you mean you plan to
> introduce a better override on the command line?  In that case speak
> up now!

The kernel command line would simply be the standard/existing memmap=
to reserve a memory range.  Then, when the platform device loads, it
does a request_firmware() to inject a binary table that further carves
memory into ranges to which the pmem driver attaches.  No need for the
legacy system BIOS to be upgraded to the "new way".

>> Our planned solution to the "legacy pmem" problem is to have a
>> userspace utility craft a list of address ranges in the form that ACPI
>> expects and attach that to a platform device (one time setup).  It
>> only requires that the memory be marked reserved, not necessarily
>> marked type-12.
>
> I can't see any benefit of that over just doign the right thing in
> kernel space.

It does do the right thing in kernel space.  The userspace utility
creates the binary table (once) that can be compiled into the platform
device driver or auto-loaded by an initrd.  The problem with a new
memmap= is that it is too coarse.  For example you can't do things
like specify a pmem range per-NUMA node.
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