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

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, May 21, 2020 at 02:52:30PM -0400, Mikulas Patocka wrote:
> 
> 
> On Thu, 21 May 2020, Dan Williams wrote:
> 
> > On Thu, May 21, 2020 at 10:03 AM Aneesh Kumar K.V
> > <aneesh.kumar@linux.ibm.com> wrote:
> > >
> > > > 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.
> > > >
> > >
> > > Correct. Thanks for catching that.
> > >
> > >
> > > I don't understand dm much, wondering how this will work with
> > > non-synchronous DAX device?
> > 
> > That's a good point. DM-writecache needs to be cognizant of things
> > like virtio-pmem that violate the rule that persisent memory writes
> > can be flushed by CPU functions rather than calling back into the
> > driver. It seems we need to always make the flush case a dax_operation
> > callback to account for this.
> 
> dm-writecache is normally sitting on the top of dm-linear, so it would 
> need to pass the wmb() call through the dm core and dm-linear target ... 
> that would slow it down ... I remember that you already did it this way 
> some times ago and then removed it.
> 
> What's the exact problem with POWER? Could the POWER system have two types 
> of persistent memory that need two different ways of flushing?

As far as I understand the discussion so far

 - on POWER $oldhardware uses $oldinstruction to ensure pmem consistency
 - on POWER $newhardware uses $newinstruction to ensure pmem consistency
   (compatible with $oldinstruction on $oldhardware)
 - on some platforms instead of barrier instruction a callback into the
   driver is issued to ensure consistency

None of this is reflected by the dm driver.

Thanks

Michal
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