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Date:   Wed, 17 Oct 2018 15:11:49 -0400 (EDT)
From:   Pankaj Gupta <pagupta@redhat.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Kevin Wolf <kwolf@redhat.com>, Jan Kara <jack@suse.cz>,
        Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
        KVM list <kvm@vger.kernel.org>,
        Rik van Riel <riel@surriel.com>,
        linux-nvdimm <linux-nvdimm@ml01.01.org>,
        David Hildenbrand <david@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Dave Jiang <dave.jiang@intel.com>,
        Qemu Developers <qemu-devel@nongnu.org>,
        Christoph Hellwig <hch@infradead.org>,
        Vishal L Verma <vishal.l.verma@intel.com>,
        Igor Mammedov <imammedo@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>, zwisler@kernel.org,
        lcapitulino@redhat.com, Paolo Bonzini <pbonzini@redhat.com>,
        Nitesh Narayan Lal <nilal@redhat.com>
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References: <20181013050021.11962-1-pagupta@redhat.com> <20181013050021.11962-3-pagupta@redhat.com> <CAPcyv4hroaVWA-HgjWCnr7QTd_y7U8sCvS+Up733ttnD6_cKzQ@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH v2 2/2] virtio-pmem: Add virtio pmem driver
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Thread-Topic: virtio-pmem: Add virtio pmem driver
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> On Fri, Oct 12, 2018 at 10:01 PM Pankaj Gupta <pagupta@redhat.com> wrote:
> >
> > This patch adds virtio-pmem driver for KVM guest.
> >
> > Guest reads the persistent memory range information from
> > Qemu over VIRTIO and registers it on nvdimm_bus. It also
> > creates a nd_region object with the persistent memory
> > range information so that existing 'nvdimm/pmem' driver
> > can reserve this into system memory map. This way
> > 'virtio-pmem' driver uses existing functionality of pmem
> > driver to register persistent memory compatible for DAX
> > capable filesystems.
> >
> > This also provides function to perform guest flush over
> > VIRTIO from 'pmem' driver when userspace performs flush
> > on DAX memory range.
> 
> Before we can move forward with this driver we need additional
> filesystem enabling to detect when the backing device is fronting DAX
> pmem or a paravirtualized page cache through virtio-pmem. Any
> interface that requires fsync() and a round trip to the hypervisor to
> flush host page cache is not DAX.

I saw your proposal[1] for new mmap flag MAP_DIRECT. IIUIC mapping should fail for 
MAP_DIRECT if it requires explicit flush or buffer indirection. So, if we disable 
MAP_SYNC flag for virtio-pmem this should fail MAP_DIRECT as well? Otherwise 
without MAP_DIRECT, virtio-pmem should be defaulted to VIRTIO flush mechanism.     

[1] https://marc.info/?l=linux-fsdevel&m=153953206330814&w=2

Thanks,
Pankaj