linux-kernel - Re: [Qemu-devel] [RFC v2 0/2] kvm "fake DAX" device flushing

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Date:   Fri, 1 Jun 2018 14:24:10 +0200
From:   Igor Mammedov <imammedo@...hat.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org, linux-nvdimm@...1.01.org,
        linux-mm@...ck.org, kwolf@...hat.com, haozhong.zhang@...el.com,
        jack@...e.cz, xiaoguangrong.eric@...il.com, riel@...riel.com,
        niteshnarayanlal@...mail.com, david@...hat.com,
        ross.zwisler@...el.com, lcapitulino@...hat.com, hch@...radead.org,
        mst@...hat.com, stefanha@...hat.com, marcel@...hat.com,
        pbonzini@...hat.com, dan.j.williams@...el.com, nilal@...hat.com
Subject: Re: [Qemu-devel] [RFC v2 0/2] kvm "fake DAX" device flushing

On Wed, 25 Apr 2018 16:54:12 +0530
Pankaj Gupta <pagupta@...hat.com> wrote:

[...]
> - Qemu virtio-pmem device
>   It exposes a persistent memory range to KVM guest which 
>   at host side is file backed memory and works as persistent 
>   memory device. In addition to this it provides virtio 
>   device handling of flushing interface. KVM guest performs
>   Qemu side asynchronous sync using this interface.
a random high level question,
Have you considered using a separate (from memory itself)
virtio device as controller for exposing some memory, async flushing.
And then just slaving pc-dimm devices to it with notification/ACPI
code suppressed so that guest won't touch them?

That way it might be more scale-able, you consume only 1 PCI slot
for controller vs multiple for virtio-pmem devices.


> Changes from previous RFC[1]:
> 
> - Reuse existing 'pmem' code for registering persistent 
>   memory and other operations instead of creating an entirely 
>   new block driver.
> - Use VIRTIO driver to register memory information with 
>   nvdimm_bus and create region_type accordingly. 
> - Call VIRTIO flush from existing pmem driver.
> 
> Details of project idea for 'fake DAX' flushing interface is 
> shared [2] & [3].
> 
> Pankaj Gupta (2):
>    Add virtio-pmem guest driver
>    pmem: device flush over VIRTIO
> 
> [1] https://marc.info/?l=linux-mm&m=150782346802290&w=2
> [2] https://www.spinics.net/lists/kvm/msg149761.html
> [3] https://www.spinics.net/lists/kvm/msg153095.html  
> 
>  drivers/nvdimm/region_devs.c     |    7 ++
>  drivers/virtio/Kconfig           |   12 +++
>  drivers/virtio/Makefile          |    1 
>  drivers/virtio/virtio_pmem.c     |  118 +++++++++++++++++++++++++++++++++++++++
>  include/linux/libnvdimm.h        |    4 +
>  include/uapi/linux/virtio_ids.h  |    1 
>  include/uapi/linux/virtio_pmem.h |   58 +++++++++++++++++++
>  7 files changed, 201 insertions(+)
>