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

[Aneesh Kumar K.V]

Dan Williams <dan.j.williams at intel.com> writes:

> On Mon, May 18, 2020 at 10:30 PM Aneesh Kumar K.V
> <aneesh.kumar at linux.ibm.com> wrote:

...

>> 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.

>
>> > 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