linux-kernel - Re: [Qemu-devel] [RFC 2/2] KVM: add virtio-pmem driver

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Message-ID: <20171018130339.GB29767@stefanha-x1.localdomain>
Date:   Wed, 18 Oct 2017 15:03:39 +0200
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>, kwolf@...hat.com,
        haozhong zhang <haozhong.zhang@...el.com>, jack@...e.cz,
        xiaoguangrong eric <xiaoguangrong.eric@...il.com>,
        kvm@...r.kernel.org, david@...hat.com, linux-nvdimm@...1.01.org,
        ross zwisler <ross.zwisler@...el.com>,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        linux-mm@...ck.org, stefanha@...hat.com, pbonzini@...hat.com,
        dan j williams <dan.j.williams@...el.com>, nilal@...hat.com
Subject: Re: [Qemu-devel] [RFC 2/2] KVM: add virtio-pmem driver

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

The virtio-pmem driver could register itself similar to the existing
ACPI NFIT and BIOS e820 pmem drivers.

Stefan