On Wed, Oct 18, 2017 at 08:51:37AM -0700, Dan Williams wrote:

On Wed, Oct 18, 2017 at 6:03 AM, Stefan Hajnoczi [off-list ref] wrote:

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On Tue, Oct 17, 2017 at 04:30:41AM -0400, Pankaj Gupta wrote:

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Are you saying do it as existing i.e ACPI pmem like interface?
The reason we have created this new driver is exiting pmem driver
does not define proper semantics for guest flushing requests.

At this point I'm caring about the Linux-internal interface, and
for that it should be integrated into the nvdimm subsystem and not
a block driver.  How the host <-> guest interface looks is a different
idea.

quoted

Regarding block support of driver, we want to achieve DAX support
to bypass guest page cache. Also, we want to utilize existing DAX
capable file-system interfaces(e.g fsync) from userspace file API's
to trigger the host side flush request.

Well, if you want to support XFS+DAX better don't make it a block
devices, because I'll post patches soon to stop using the block device
entirely for the DAX case.

o.k I will look at your patches once they are in mailing list.
Thanks for the heads up.

If I am guessing it right, we don't need block device additional features
for pmem? We can bypass block device features like blk device cache flush etc.
Also, still we would be supporting ext4 & XFS filesystem with pmem?

If there is time to your patches can you please elaborate on this a bit.

I think the idea is that the nvdimm subsystem already adds block device
semantics on top of the struct nvdimms that it manages.  See
drivers/nvdimm/blk.c.

So it would be cleaner to make virtio-pmem an nvdimm bus.  This will
eliminate the duplication between your driver and drivers/nvdimm/ code.
Try "git grep nvdimm_bus_register" to find drivers that use the nvdimm
subsystem.

This use case is not "Persistent Memory". Persistent Memory is
something you can map and make persistent with CPU instructions.
Anything that requires a driver call is device driver managed "Shared
Memory".

Dan, in that case do you have ideas regarding Christoph Hellwig's
comment that this driver should be integrated into the nvdimm subsystem
instead of a new block driver?

Stefan

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