Re: KVM "fake DAX" flushing interface - discussion
On Wed, Jul 26, 2017 at 2:27 PM, Rik van Riel <riel(a)redhat.com> 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