linux-kernel - Re: [PATCH v2 2/3] libnvdimm, pmem: adjust for section collisions with 'System RAM'

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Date:	Mon, 7 Mar 2016 09:18:42 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Toshi Kani <toshi.kani@....com>
Cc:	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] libnvdimm, pmem: adjust for section collisions
 with 'System RAM'

On Mon, Mar 7, 2016 at 9:56 AM, Toshi Kani <toshi.kani@....com> wrote:
> On Fri, 2016-03-04 at 18:23 -0800, Dan Williams wrote:
>> On Fri, Mar 4, 2016 at 6:48 PM, Toshi Kani <toshi.kani@....com> wrote:
[..]
>> As far as I can see
>> all we do is ask firmware implementations to respect Linux section
>> boundaries and otherwise not change alignments.
>
> In addition to the requirement that pmem range alignment may not change,
> the code also requires a regular memory range does not change to intersect
> with a pmem section later.  This seems fragile to me since guest config may
> vary / change as I mentioned above.
>
> So, shouldn't the driver fails to attach when the range is not aligned by
> the section size?  Since we need to place a requirement to firmware anyway,
> we can simply state that it must be aligned by 128MiB (at least) on x86.
>  Then, memory and pmem physical layouts can be changed as long as this
> requirement is met.

We can state that it must be aligned, but without a hard specification
I don't see how we can guarantee it.  We will fail the driver load
with a warning if our alignment fixups end up getting invalidated by a
later configuration change, but in the meantime we cover the gap of a
BIOS that has generated a problematic configuration.