Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion

[Dan Williams]

On Wed, Jul 26, 2017 at 2:27 PM, Rik van Riel <address@hidden> wrote:
> On Wed, 2017-07-26 at 09:47 -0400, Pankaj Gupta wrote:
>> >
>> Just want to summarize here(high level):
>>
>> This will require implementing new 'virtio-pmem' device which
>> presents
>> a DAX address range(like pmem) to guest with read/write(direct
>> access)
>> & device flush functionality. Also, qemu should implement
>> corresponding
>> support for flush using virtio.
>>
> Alternatively, the existing pmem code, with
> a flush-only block device on the side, which
> is somehow associated with the pmem device.
>
> I wonder which alternative leads to the least
> code duplication, and the least maintenance
> hassle going forward.

I'd much prefer to have another driver. I.e. a driver that refactors
out some common pmem details into a shared object and can attach to
ND_DEVICE_NAMESPACE_{IO,PMEM}. A control device on the side seems like
a recipe for confusion.

With a $new_driver in hand you can just do:

   modprobe $new_driver
   echo $namespace > /sys/bus/nd/drivers/nd_pmem/unbind
   echo $namespace > /sys/bus/nd/drivers/$new_driver/new_id
   echo $namespace > /sys/bus/nd/drivers/$new_driver/bind

...and the guest can arrange for $new_driver to be the default, so you
don't need to do those steps each boot of the VM, by doing:

    echo "blacklist nd_pmem" > /etc/modprobe.d/virt-dax-flush.conf
    echo "alias nd:t4* $new_driver" >> /etc/modprobe.d/virt-dax-flush.conf
    echo "alias nd:t5* $new_driver" >> /etc/modprobe.d/virt-dax-flush.conf