Re: [Linux-nvdimm] [PATCH v2 11/20] libnd, nd_pmem: add libnd support to the pmem driver

From: Andy Lutomirski
Date: Tue Apr 28 2015 - 20:29:03 EST

On Tue, Apr 28, 2015 at 5:17 PM, Phil Pokorny
<ppokorny@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Apr 28, 2015 at 3:58 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Tue, Apr 28, 2015 at 3:21 PM, Phil Pokorny
>> <ppokorny@xxxxxxxxxxxxxxxxxxxx> wrote:
>>> On Tue, Apr 28, 2015 at 2:04 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>> On Tue, Apr 28, 2015 at 11:25 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>>>>> +config ND_E820
>>>>> + tristate "E820: Support the E820-type-12 PMEM convention"
>>>>> + depends on X86_PMEM_LEGACY
>>>>> + default m if X86_PMEM_LEGACY
>>>>> + select LIBND
>>>>> + help
>>>>> + Prior to ACPI 6 some platforms advertised peristent memory
>>>>> + via type-12 e820 memory ranges. Create a libnd bus and
>>>>> + attach an instance of the pmem driver to these ranges.
>>>>> +
>>>> How about something like:
>>>> "This driver allows libnd to work with legacy, pre-ACPI 6 NVDIMMs.
>>>> This enables such devices to be exposed as block devices using PMEM.
>>>> The legacy NVDIMM interface is problematic. This driver will not work
>>>> if you boot using UEFI, and some NVDIMMs and motherboards that work
>>>> with this driver may require proprietary code in order to work
>>>> reliably."
>>> Perhaps not "problematic" but "requires a BIOS in Legacy mode"
>>> It might also mention that if you use the kernel command line
>>> memmap=nn!ss syntax it adds
>>> a type 12 region to the e820 map and so you would want this support.
>>> If you have a motherboard with UEFI support for NVDIMM's that would be
>>> the recommended
>>> configuration.
>> This is such a mess that I think this driver should maybe flat-out
>> refuse to load in this type of configuration without some scary module
>> option. I have some NVDIMMs that report as type 12 but need two extra
>> out-of-tree drivers to work safely. First, they need i2c_imc or the
>> equivalent (I'll try to resubmit that soon). Second, they need secret
>> magic NDAed register poking. The latter is very problematic.
> My current experience is that things may be changing to something of a de-facto
> standard in the area of register poking. In which case, we should be
> able to ask
> the de-facto vendor standard to be released under a non-NDA license so we can
> write a proper user-space library for it. Or at worst, get a
> proprietary source utility
> that can do the poking.
> The vendor isn't going to sell anything if they don't provide the
> tools their resellers
> and customers need.

I suspect that the vendor will soon be done selling this particular
part as they move toward something more standard. Dunno.

>> At the very least, I think we should discourage people who don't
>> really know what they're doing from using this driver without care.
> What would be the fun in that...
> But seriously, speaking as Penguin Computing and a retailer of
> hardware, I'd rather
> not have the kernel telling my customers what's safe and what isn't
> when it's a matter
> of opinion. We provide a solution with support and having to tell my
> customers: "you
> need to load the module with the 'THIS_IS_UNSAFE' argument set to 3"
> isn't productive.

It could be that you load with i_promise_i_have_an_nvdimm_driver_too=1
or, better yet, if loaded without the magic option but with the magic
driver it figures it out and initializes anyway.

> Another intersesting possibility of the memmap= directive to declare a
> type 12 region of
> of memory is that you can test the driver (without the persistance) on
> any arbitrary region
> of memory in a machine. Other comments on this patch set talked about
> having to put
> virtual test hardware in qemu or kvm. Aside from the register poking,
> just adding
> memmap=xx!yy to the command line gives you something pmem can attach to and
> you can use to test with. I suppose you could even simulate
> persistance by saving off
> the contents and restoring it on a controlled reboot.

That's definitely useful.

Anyway, I don't object strongly to the driver as is. Anyone with a
legacy NVDIMM is already dependent on all kinds of things going right
(correct power supply, ADR, all pins wired correctly, correct BIOS
version, no EFI, lack of pcommit not being a problem, etc).

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