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

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