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From: Jeff Moyer <jmoyer@redhat.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
 Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 3/5] libnvdimm/nvdimm/flush: Allow architecture to
 override the flush barrier
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 (Dan Williams's message of "Tue, 19 May 2020 11:59:30 -0700")
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Dan Williams <dan.j.williams@intel.com> writes:

>> But I agree with your concern that if we have older kernel/applications
>> that continue to use `dcbf` on future hardware we will end up
>> having issues w.r.t powerfail consistency. The plan is what you outlined
>> above as tighter ecosystem control. Considering we don't have a pmem
>> device generally available, we get both kernel and userspace upgraded
>> to use these new instructions before such a device is made available.

I thought power already supported NVDIMM-N, no?  So are you saying that
those devices will continue to work with the existing flushing and
fencing mechanisms?

> Ok, I think a compile time kernel option with a runtime override
> satisfies my concern. Does that work for you?

The compile time option only helps when running newer kernels.  I'm not
sure how you would even begin to audit userspace applications (keep in
mind, not every application is open source, and not every application
uses pmdk).  I also question the merits of forcing the administrator to
make the determination of whether all applications on the system will
work properly.  Really, you have to rely on the vendor to tell you the
platform is supported, and at that point, why put further hurdles in the
way?

The decision to require different instructions on ppc is unfortunate,
but one I'm sure we have no control over.  I don't see any merit in the
kernel disallowing MAP_SYNC access on these platforms.  Ideally, we'd
have some way of ensuring older kernels don't work with these new
platforms, but I don't think that's possible.

Moving on to the patch itself--Aneesh, have you audited other persistent
memory users in the kernel?  For example, drivers/md/dm-writecache.c does
this:

static void writecache_commit_flushed(struct dm_writecache *wc, bool wait_for_ios)
{
 	if (WC_MODE_PMEM(wc))
	        wmb(); <==========
        else
                ssd_commit_flushed(wc, wait_for_ios);
}

I believe you'll need to make modifications there.

Cheers,
Jeff