Re: [PATCH v4 3/6] libnvdimm, acpi, nfit: Add bus level dsm mask for pass thru.

[Jerry Hoemann]

On Wed, Jul 05, 2017 at 09:35:48AM -0700, Dan Williams wrote:
> On Wed, Jul 5, 2017 at 9:24 AM, Jerry Hoemann <jerry.hoemann@xxxxxxx> wrote:
> > On Tue, Jul 04, 2017 at 01:37:43PM -0700, Dan Williams wrote:
> >> On Tue, Jul 4, 2017 at 1:08 PM, Jerry Hoemann <jerry.hoemann@xxxxxxx> wrote:
> >> > On Sat, Jul 01, 2017 at 01:46:03PM -0700, Dan Williams wrote:
> >> >> On Sat, Jul 1, 2017 at 1:38 PM, Jerry Hoemann <jerry.hoemann@xxxxxxx> wrote:
> >> >> > On Sat, Jul 01, 2017 at 01:10:31PM -0700, Dan Williams wrote:
> >> >> >> On Sat, Jul 1, 2017 at 1:08 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >> >> >> > On Sat, Jul 1, 2017 at 12:58 PM, Jerry Hoemann <jerry.hoemann@xxxxxxx> wrote:
> >> >> >> >> On Fri, Jun 30, 2017 at 08:55:22PM -0700, Dan Williams wrote:
> >> >> >> >>
> >> >> >> >> ...
> >
> > ...
> >
> >> >> >> >>>
> >> >> >> >>> This drops function number 0 which userspace has no need to call.
> >> >> >> >>
> >> >> >> >> Actually I like to call function 0.  Its an excellent test when
> >> >> >> >> modifying the code path as its a no side effects function whose output
> >> >> >> >> is known in advance and instantly recognizable.  I also use it when
> >> >> >> >> testing new firmware.
> >> >> >> >>
> >> >> >> >> What is the downside to allowing it?  What bad things happen?
> >> >> >> >
> >> >> >> > It allows implementations to bypass the standardization process and
> >> >> >> > ship new root DSMs. It's always possible to patch the kernel locally
> >> >> >> > for development, so I see no reason to ship this capability globally.
> >> >> >
> >> >> > I don't understand this comment, but I think your next comment
> >> >> > essentially says to disregard this comment?
> >> >>
> >> >> Yes, sorry.
> >> >>
> >> >> >> Actually, just the discovery portion does not lead to this leak, but
> >> >> >> it's redundant when we have the 'dsm_mask' sysfs attribute.
> >> >> >
> >> >> > No.  The generation of the mask in sysfs is not done by
> >> >> > executing the code in acpi_nfit_ctl.  One of the reasons I call
> >> >> > function 0 to test changes I am making to the ioctl path itself.
> >> >> > The sysfs has nothing to do with that path and cannot be used
> >> >> > to serve this purpose.
> >> >> >
> >> >> > And since the content of sysfs has been edited it also can not be
> >> >> > used as a basic test of firmware.
> >> >> >
> >> >> > What is the downside to allowing the calling of function 0?
> >> >>
> >> >> It needlessly expands the kernel ABI. I would suggest, if you want to
> >> >
> >> > No.  It is not needless.  It is not an ABI extension.
> >> > Same goes for the override feature.
> >>
> >> If the need is testing then we have a tools/testing/nvdimm for that.
> >
> >
> >
> >> Of course it's an ABI extension, it allows userspace to discover DSM
> >> function numbers the kernel didn't know about at compile time.
> >
> >
> > A modification to a library or kernel that changes the results of a
> > function (or system call) doesn't necessarily break (or extend) an ABI.
> > An obvious example is that of a random number generator function.
> > A library/kernel is completely free to change the implementation
> > of the random number generator (and the values it returns)
> > without breaking the ABI provided all other rules of ABI preservation
> > are followed.
> >
> > Now lets look at problem at hand.  The pass thru mechanism has very
> > little semantic overhead.  Fill in the nd_cmd_pkg as described in ndctl.h,
> > call the ioctl w/ argument with ND_CMD_CALL, and the kernel will marshal
> > up the arguments, call the DSM and return the results.  The values
> > of nd_command could be any value and it is for the DSM to either accept
> > or reject the input argument.  I wrote this interface and this is how
> > I defined it.
> >
> > The user application is not changing irrespective of if the kernel applies
> > a mask to the passed in nd_command argument.  The data structures are not
> > changing at either source level or binary level. The calling convention is not
> > changing.  No object file changes are required.  Nothing related to ABI
> > preservation is impacted.  The only question is whether the application
> > of a mask to special case function 0 breaks/extends the ABI.
> >
> > It turns out that this point doesn't really matter as your position
> > is invalid either way.
> >
> > The argument for this not being an API breakage/extension:
> >
> > A DSM could either implement or not a function index for any value of N.
> > So, a correctly written application must take into account that for
> > any value of N, the DSM may return error or not.  Preserving an ABI
> > doesn't require the library/kernel preserve incorrect application
> > behavior.
> >
> > Now, assume that the special casing of function zero does constitute
> > a breakage/extension of the ABI:
> >
> > I'm not the one wishing to special case function 0, you are.
> > So, to this point I say, Dan please don't make needless extension to
> > the ABI. Its and extension and you've  provided no valid reason
> > for making it.
> >
> > Your argument to disallow function zero is invalid.
> >
> > There is nothing harmful per se to allow function 0.  All DSMs that return
> > non zero are required to have it. By excluding it, you actually create the
> > impression that the underlying DSM is violating the DSM specification.
> 
> This goes back to the original reasoning for pushing back on the
> override for the leaf-level _DSM methods. Specifically the ability to
> bypass the standardization process to ship vendor-specific behavior.

You're conflating the two contested patches.  Allowing function 0 and
allowing the override.

While similar in most respects, function index 0 and function index != 0,
there is a key difference,  function 0 is defined by ACPI for all DSMs.
So, there can be no bypassing of standardization process with function 0
as it is already defined.

As for the addition of new DSM functions in the future, remember that
the DSM is governed by a guid that is defined in the ACPI spec.  While
it is technically true that any one who writes firmware could create
firmware that hijacks the DSM interface to add new functions not currently
defined it would be foolish for them to do so as they risk collisions with
updates from the ACPI forum.  (They could also modify already defined
and allowed by linux functions.  But again, foolish.)



> Now, the other side of the argument is that if the next spec adds new
> _DSMs a simple override can enable them. I am more sympathetic to the
> override for the leaf / DIMM level because those _DSMs truly are
> DIMM-vendor specific, but the root device is not. Also, none of the
> root-level DSMs added for 6.2 are in any way critical for proper
> operation of the platform, and I do not see any bus-level
> functionality on the horizon that we need to aggressively pre-enable.


> It was a mistake to use _DSM for common root-level functionality, and
> we shouldn't double down on that mistake by allowing unfettered

As to the moral aspects of ACPI's decision to standardiz the DSM for NVDIMM,
I take no position on whether it was a good thing or a bad thing; but it
is a thing.  We need to handle it.  I see no particular benefit to
making our own lives more difficult.


> definition of new interfaces. NVDIMM is not so special that it needs
> to bypass the standard ACPI-to-kernel development pipeline.

-- 

-----------------------------------------------------------------------------
Jerry Hoemann                  Software Engineer   Hewlett Packard Enterprise
-----------------------------------------------------------------------------