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 for <linux-nvdimm@lists.01.org>; Wed, 17 Apr 2019 23:27:03 -0700 (PDT)
Date: Thu, 18 Apr 2019 02:27:00 -0400 (EDT)
From: Pankaj Gupta <pagupta@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v5 1/6] libnvdimm: nd_region flush callback
 support
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To: Jeff Moyer <jmoyer@redhat.com>, Jan Kara <jack@suse.cz>, Dan Williams <dan.j.williams@intel.com>
Cc: KVM list <kvm@vger.kernel.org>, "Michael S. Tsirkin" <mst@redhat.com>, Jason Wang <jasowang@redhat.com>, david <david@fromorbit.com>, Qemu Developers <qemu-devel@nongnu.org>, virtualization@lists.linux-foundation.org, Andreas Dilger <adilger.kernel@dilger.ca>, Ross Zwisler <zwisler@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>, linux-nvdimm <linux-nvdimm@lists.01.org>, David Hildenbrand <david@redhat.com>, Matthew Wilcox <willy@infradead.org>, Christoph Hellwig <hch@infradead.org>, Linux ACPI <linux-acpi@vger.kernel.org>, linux-ext4 <linux-ext4@vger.kernel.org>, Len Brown <lenb@kernel.org>, kilobyte@angband.pl, Rik van Riel <riel@surriel.com>, yuval shaia <yuval.shaia@oracle.com>, Stefan Hajnoczi <stefanha@redhat.com>, Igor Mammedov <imammedo@redhat.com>, lcapitulino@redhat.com, Kevin Wolf <kwolf@redhat.com>, Nitesh Narayan Lal <nilal@redhat.com>, Theodore Ts'o <tytso@mit.edu>, Xiao Guangrong <xiaoguangrong.eric@gmail.com>, cohuck@redhat.com, "Rafael J. Wysocki" <rjw@rjwysocki.net>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-xfs <linux-xfs@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Paolo Bonzini <pbonzini@redhat.com>, "Darrick J. Wong" <darrick.wong@oracle.com>
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Hello,

Thank you for the suggestions on this.

> 
> > 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.

Can we add the optimization after this version is merged.

Best regards,
Pankaj 

> 
> -Jeff
> 
> 
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