linux-kernel - Re: [RFC 2/2] KVM: add virtio-pmem driver

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Date:   Thu, 12 Oct 2017 18:18:39 -0400 (EDT)
From:   Pankaj Gupta <pagupta@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-kernel@...r.kernel.org, KVM list <kvm@...r.kernel.org>,
        Qemu Developers <qemu-devel@...gnu.org>,
        linux-nvdimm <linux-nvdimm@...1.01.org>,
        Linux MM <linux-mm@...ck.org>, Jan Kara <jack@...e.cz>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Haozhong Zhang <haozhong.zhang@...el.com>,
        Nitesh Narayan Lal <nilal@...hat.com>,
        Kevin Wolf <kwolf@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ross Zwisler <ross.zwisler@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [RFC 2/2] KVM: add virtio-pmem driver


> 
> On Thu, Oct 12, 2017 at 2:25 PM, Pankaj Gupta <pagupta@...hat.com> wrote:
> >
> >> >   This patch adds virtio-pmem driver for KVM guest.
> >> >   Guest reads the persistent memory range information
> >> >   over virtio bus from Qemu and reserves the range
> >> >   as persistent memory. Guest also allocates a block
> >> >   device corresponding to the pmem range which later
> >> >   can be accessed with DAX compatible file systems.
> >> >   Idea is to use the virtio channel between guest and
> >> >   host to perform the block device flush for guest pmem
> >> >   DAX device.
> >> >
> >> >   There is work to do including DAX file system support
> >> >   and other advanced features.
> >> >
> >> > Signed-off-by: Pankaj Gupta <pagupta@...hat.com>
> >> > ---
> >> >  drivers/virtio/Kconfig           |  10 ++
> >> >  drivers/virtio/Makefile          |   1 +
> >> >  drivers/virtio/virtio_pmem.c     | 322
> >> >  +++++++++++++++++++++++++++++++++++++++
> >> >  include/uapi/linux/virtio_pmem.h |  55 +++++++
> >> >  4 files changed, 388 insertions(+)
> >> >  create mode 100644 drivers/virtio/virtio_pmem.c
> >> >  create mode 100644 include/uapi/linux/virtio_pmem.h
> >> >
> >> > diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> >> > index cff773f15b7e..0192c4bda54b 100644
> >> > --- a/drivers/virtio/Kconfig
> >> > +++ b/drivers/virtio/Kconfig
> >> > @@ -38,6 +38,16 @@ config VIRTIO_PCI_LEGACY
> >> >
> >> >           If unsure, say Y.
> >> >
> >> > +config VIRTIO_PMEM
> >> > +       tristate "Virtio pmem driver"
> >> > +       depends on VIRTIO
> >> > +       ---help---
> >> > +        This driver adds persistent memory range within a KVM guest.
> >>
> >> I think we need to call this something other than persistent memory to
> >> make it clear that this not memory where the persistence can be
> >> managed from userspace. The persistence point always requires a driver
> >> call, so this is something distinctly different than "persistent
> >> memory". For example, it's a bug if this memory range ends up backing
> >> a device-dax range in the guest where there is no such thing as a
> >> driver callback to perform the flushing. How does this solution
> >> protect against that scenario?
> >
> > yes, you are right we are not providing device_dax in this case so it
> > should
> > be clear from name. Any suggestion for name?
> 
> So currently /proc/iomem in a guest with a pmem device attached to a
> namespace looks like this:
> 
>     c00000000-13bfffffff : Persistent Memory
>        c00000000-13bfffffff : namespace2.0
> 
> Can we call it "Virtio Shared Memory" to make it clear it is a
> different beast than typical "Persistent Memory"?  You can likely

I think somewhere we need persistent keyword 'Virtio Persistent Memory' or 
so.

> inject your own name into the resource tree the same way we do in the
> NFIT driver. See acpi_nfit_insert_resource().

Sure! thank you.