From: Dan Williams <dan.j.williams@intel.com>
To: Pankaj Gupta <pagupta@redhat.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>
Subject: Re: [Qemu-devel] [PATCH v2 2/2] virtio-pmem: Add virtio pmem driver
Date: Wed, 17 Oct 2018 12:36:51 -0700 [thread overview]
Message-ID: <CAPcyv4joz_dPruJY4CYAxXSuMTD-NUoqm6kvGFrF19jt5dN=rg@mail.gmail.com> (raw)
In-Reply-To: <431127218.21694133.1539803509205.JavaMail.zimbra@redhat.com>
On Wed, Oct 17, 2018 at 12:11 PM Pankaj Gupta <pagupta@redhat.com> wrote:
>
>
>
> > 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.
Right, although I wouldn't worry about MAP_DIRECT in the short term
since we're still discussing what the upstream interface. Regardless
of whether MAP_DIRECT is specified or not the virtio-flush mechanism
would always be used for virtio-pmem. I.e. there is no possibility to
get full DAX operation with virtio-pmem, only the page-cache bypass
sub-set.
Taking a look at where we could inject this check for filesystems it's
a bit awkward to do it in xfs_file_mmap() for example because we do
not have the backing device for the extents of the inode. So at a
minimum you would need to investigate calling xfs_inode_supports_dax()
from that path and teaching it about a new dax_device flag. I'm
thinking the dax_device flag should be called DAXDEV_BUFFERED to
indicate the presence of software buffering on a device that otherwise
supports bypassing the local page cache.
next prev parent reply other threads:[~2018-10-17 19:36 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-13 5:00 [PATCH v2 0/2] kvm "fake DAX" device Pankaj Gupta
[not found] ` <20181013050021.11962-1-pagupta-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2018-10-13 5:00 ` [PATCH v2 1/2] libnvdimm: nd_region flush callback support Pankaj Gupta
2018-10-13 8:31 ` kbuild test robot
[not found] ` <20181013050021.11962-2-pagupta-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2018-10-13 9:38 ` kbuild test robot
2018-10-13 16:06 ` [PATCH v2 0/2] kvm "fake DAX" device Dan Williams
2018-10-13 16:29 ` [Qemu-devel] " Pankaj Gupta
2018-10-13 5:00 ` [PATCH v2 2/2] virtio-pmem: Add virtio pmem driver Pankaj Gupta
[not found] ` <20181013050021.11962-3-pagupta-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2018-10-13 16:10 ` Dan Williams
[not found] ` <CAPcyv4hroaVWA-HgjWCnr7QTd_y7U8sCvS+Up733ttnD6_cKzQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-10-17 19:11 ` [Qemu-devel] " Pankaj Gupta
2018-10-17 19:36 ` Dan Williams [this message]
2018-10-18 1:44 ` Pankaj Gupta
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