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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Sat, 13 Oct 2018 09:10:13 -0700
Message-ID: <CAPcyv4hroaVWA-HgjWCnr7QTd_y7U8sCvS+Up733ttnD6_cKzQ@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] virtio-pmem: Add virtio pmem driver
To:     Pankaj Gupta <pagupta@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>,
        Qemu Developers <qemu-devel@nongnu.org>,
        linux-nvdimm <linux-nvdimm@ml01.01.org>, Jan Kara <jack@suse.cz>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Rik van Riel <riel@surriel.com>,
        Nitesh Narayan Lal <nilal@redhat.com>,
        Kevin Wolf <kwolf@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>, zwisler@kernel.org,
        Vishal L Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>, lcapitulino@redhat.com,
        Igor Mammedov <imammedo@redhat.com>,
        Eric Blake <eblake@redhat.com>
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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.