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

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Date:   Tue, 17 Oct 2017 01:02:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
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 03:40:56AM -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.