linux-kernel - Re: [PATCH v2 0/3] nvdimm: Add an IOCTL pass thru for DSM calls

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Date:	Mon, 16 Nov 2015 17:29:41 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Jerry Hoemann <Jerry.Hoemann@....com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	"Elliott, Robert (Persistent Memory)" <elliott@....com>,
	jmoyer <jmoyer@...hat.com>,
	Dmitry Krivenok <krivenok.dmitry@...il.com>,
	Linda Knippers <linda.knippers@....com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] nvdimm: Add an IOCTL pass thru for DSM calls

On Mon, Nov 16, 2015 at 1:10 PM, Jerry Hoemann <jerry.hoemann@....com> wrote:
> On Mon, Nov 16, 2015 at 11:00:20AM -0800, Dan Williams wrote:
>> On Mon, Nov 16, 2015 at 10:38 AM, Jerry Hoemann <jerry.hoemann@....com> wrote:
>> >
>> > The NVDIMM code in the kernel supports an IOCTL interface to user
>> > space based upon the Intel Example DSM:
>> >
>> >         http://pmem.io/documents/NVDIMM_DSM_Interface_Example.pdf
>> >
>> > This interface cannot be used by other NVDIMM DSMs that support
>> > incompatible functions.
>> >
>> > This patch set adds a generic "passthru" IOCTL interface which
>> > is not tied to a particular DSM.
>> >
>> > A new _IOC_NR ND_CMD_PASSTHRU == "100" is added for the pass thru call.
>> >
>> > The new data structure nd_passthru_pkg serves as a wrapper for the passthru
>> > calls.  This wrapper supplies the data that the kernel needs to
>> > make the _DSM call.
>> >
>> > Unlike the definitions of the _DSM functions themselves, the nd_passthru_pkg
>> > provides the calling information (input/output sizes) in an uniform
>> > manner making the kernel marshaling of the arguments straight
>> > forward.
>> >
>> > This shifts the marshaling burden from the kernel to the user
>> > space application while still permitting the kernel to internally
>> > call _DSM functions.
>> >
>> > To make the resultant kernel code easier to understand the existing
>> > functions acpi_nfit_ctl and __nd_ioctl were renamed to .*_intel to
>> > denote calling mechanism as in 4.3 tailored to the Intel Example DSM.
>> > New functions acpi_nfit_ctl_passthru and __nd_ioctl_passthru were
>> > created to supply the pass thru interface.
>>
>> Let's not do the _intel vs _passthru split.  I want to convert the
>> existing commands over to this new interface and deprecate the old
>> ioctl-command formats.  I.e. it isn't the case that this will be a
>> always be a blind "passthru" mechanism, the kernel will need to crack
>> open this payload in some circumstances.
>
>
> I'm confused.
>
> In this version there is only 1 ioctl 'N'.  The pass thru is using
> number 100.  This is what I thought you wanted from prior comments.

It is indeed, I like that change.

> The split are for internal functions that deal specifically w/
> the argument marshaling code and copy-in/copy-out.  These mechanisms
> are different.
>
> I understand that you want to switch over, but don't you (at least for
> the time being) need to keep the old marshaling code for the current
> use case?   I was assuming a sequence like:
>         1. The pass thru code gets submitted.
>         2. The current tools are converted over to using the pass thru,
>         3. The marshaling code using nd_cmd_in_size etc., would then
>                 be removed.
>
> Are you wanting to make one big change and not in separate steps?

I want to do it in separate steps, I'd just like to see cmd number 100
added to the existing __nd_ioctl and acpi_nfit_ctl routines.  That
plus quibbling about the name "ND_CMD_PASSTHRU".  Given the plans to
eventually replace the existing commands we can call it something like
'ND_DSM_GENERIC'.

> Now I don't currently have provision for "snooping" out specific functions
> in the pass thru flow. The 4.3 code doesn't do that either per se.  It
> will do the calls asked of it from the user.
>
> Snooping is a feature that could be added, but since I'm not sure your
> plans here, the best I can do at the moment is to put in a hook to
> a function that is a  no-op.  The meat of this function to be added
> when plans become more firm.  Do you have more specific info here?

No need to add the snooping support now, I only made the comment in
reference to the "passthru" name.  We can add snooping support later
after we finalize this initial marshaling code.
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