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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 18 Apr 2019 09:05:05 -0700
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Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support
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To: Jeff Moyer <jmoyer@redhat.com>
Cc: Jan Kara <jack@suse.cz>, Pankaj Gupta <pagupta@redhat.com>, linux-nvdimm <linux-nvdimm@lists.01.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, virtualization@lists.linux-foundation.org, KVM list <kvm@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux ACPI <linux-acpi@vger.kernel.org>, Qemu Developers <qemu-devel@nongnu.org>, linux-ext4 <linux-ext4@vger.kernel.org>, linux-xfs <linux-xfs@vger.kernel.org>, Ross Zwisler <zwisler@kernel.org>, Vishal L Verma <vishal.l.verma@intel.com>, Dave Jiang <dave.jiang@intel.com>, "Michael S. Tsirkin" <mst@redhat.com>, Jason Wang <jasowang@redhat.com>, Matthew Wilcox <willy@infradead.org>, "Rafael J. Wysocki" <rjw@rjwysocki.net>, Christoph Hellwig <hch@infradead.org>, Len Brown <lenb@kernel.org>, Theodore Ts'o <tytso@mit.edu>, Andreas Dilger <adilger.kernel@dilger.ca>, "Darrick J. Wong" <darrick.wong@oracle.com>, lcapitulino@redhat.com, Kevin Wolf <kwolf@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Nitesh Narayan Lal <nilal@redhat.com>, Rik van Riel <riel@surriel.com>, Stefan Hajnoczi <stefanha@redhat.com>, Andrea Arcangeli <aarcange@redhat.com>, David Hildenbrand <david@redhat.com>, david <david@fromorbit.com>, cohuck@redhat.com, Xiao Guangrong <xiaoguangrong.eric@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, kilobyte@angband.pl, yuval shaia <yuval.shaia@oracle.com>

On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer <jmoyer@redhat.com> wrote:
>
> Jan Kara <jack@suse.cz> writes:
>
> > On Thu 11-04-19 07:51:48, Dan Williams wrote:
> >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta@redhat.com> wrote:
> >> > +       } else {
> >> > +               if (nd_region->flush(nd_region))
> >> > +                       rc = -EIO;
> >>
> >> Given the common case wants to be fast and synchronous I think we
> >> should try to avoid retpoline overhead by default. So something like
> >> this:
> >>
> >> if (nd_region->flush == generic_nvdimm_flush)
> >>     rc = generic_nvdimm_flush(...);
> >
> > I'd either add a comment about avoiding retpoline overhead here or just
> > make ->flush == NULL mean generic_nvdimm_flush(). Just so that people don't
> > get confused by the code.
>
> Isn't this premature optimization?  I really don't like adding things
> like this without some numbers to show it's worth it.

I don't think it's premature given this optimization technique is
already being deployed elsewhere, see:

https://lwn.net/Articles/774347/