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Subject: Re: [Qemu-devel] [RFC v2 0/2] kvm "fake DAX" device flushing
To:     Igor Mammedov <imammedo@redhat.com>,
        Pankaj Gupta <pagupta@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        qemu-devel@nongnu.org, linux-nvdimm@ml01.01.org,
        linux-mm@kvack.org, kwolf@redhat.com, haozhong.zhang@intel.com,
        jack@suse.cz, xiaoguangrong.eric@gmail.com, riel@surriel.com,
        niteshnarayanlal@hotmail.com, ross.zwisler@intel.com,
        lcapitulino@redhat.com, hch@infradead.org, mst@redhat.com,
        stefanha@redhat.com, marcel@redhat.com, pbonzini@redhat.com,
        dan.j.williams@intel.com, nilal@redhat.com
References: <20180425112415.12327-1-pagupta@redhat.com>
 <20180601142410.5c986f13@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <a36137df-d2f6-39d5-318e-9006415b8ca2@redhat.com>
Date:   Mon, 4 Jun 2018 11:55:33 +0200
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On 01.06.2018 14:24, Igor Mammedov wrote:
> On Wed, 25 Apr 2018 16:54:12 +0530
> Pankaj Gupta <pagupta@redhat.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?

I don't think slaving pc-dimm would be the right thing to do (e.g.
slots, pcdimm vs nvdimm, bus(less), etc..). However the general idea is
interesting for virtio-pmem (as we might have a bigger number of disks).

We could have something like a virtio-pmem-bus to which you attach
virtio-pmem devices. By specifying the mapping, e.g. the thread that
will be used for async flushes will be implicit.

> 
> 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(+)
>>
> 


-- 

Thanks,

David / dhildenb