Re: [PATCH v2 3/5] libnvdimm/nvdimm/flush: Allow architecture to override the flush barrier

Dan Williams [off-list ref] writes:

On Mon, May 18, 2020 at 10:30 PM Aneesh Kumar K.V
[off-list ref] wrote:

...

quoted

Applications using new instructions will behave as expected when running
on P8 and P9. Only future hardware will differentiate between 'dcbf' and
'dcbfps'

Right, this is the problem. Applications using new instructions behave
as expected, the kernel has been shipping of_pmem and papr_scm for
several cycles now, you're saying that the DAX applications written
against those platforms are going to be broken on P8 and P9?

The expecation is that both kernel and userspace would get upgraded to
use the new instruction before actual persistent memory devices are
made available.

quoted

quoted

I'm thinking the kernel
should go as far as to disable DAX operation by default on new
hardware until userspace asserts that it is prepared to switch to the
new implementation. Is there any other way to ensure the forward
compatibility of deployed ppc64 DAX applications?

AFAIU there is no released persistent memory hardware on ppc64 platform
and we need to make sure before applications get enabled to use these
persistent memory devices, they should switch to use the new
instruction?

Right, I want the kernel to offer some level of safety here because
everything you are describing sounds like a flag day conversion. Am I
misreading? Is there some other gate that prevents existing users of
of_pmem and papr_scm from having their expectations violated when
running on P8 / P9 hardware? Maybe there's tighter ecosystem control
that I'm just not familiar with, I'm only going off the fact that the
kernel has shipped a non-zero number of NVDIMM drivers that build with
ARCH=ppc64 for several cycles.

If we are looking at adding changes to kernel that will prevent a kernel
from running on newer hardware in a specific case, we could as well take
the changes to get the kernel use the newer instructions right?

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.

-aneesh