From mboxrd@z Thu Jan  1 00:00:00 1970
From: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: KVM "fake DAX" flushing interface - discussion
Date: Thu, 23 Nov 2017 12:05:45 +0800
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Cc: Pankaj Gupta <pagupta@redhat.com>, Jan Kara <jack@suse.cz>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Stefan Hajnoczi <stefanha@gmail.com>,
        kvm-devel <kvm@vger.kernel.org>,
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        Nitesh Narayan Lal <nilal@redhat.com>,
        Haozhong Zhang <haozhong.zhang@intel.com>,
        Ross Zwisler <ross.zwisler@intel.com>
To: Rik van Riel <riel@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>
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On 11/22/2017 02:19 AM, Rik van Riel wrote:

> We can go with the "best" interface for what
> could be a relatively slow flush (fsync on a
> file on ssd/disk on the host), which requires
> that the flushing task wait on completion
> asynchronously.

I'd like to clarify the interface of "wait on completion
asynchronously" and KVM async page fault a bit more.

Current design of async-page-fault only works on RAM rather
than MMIO, i.e, if the page fault caused by accessing the
device memory of a emulated device, it needs to go to
userspace (QEMU) which emulates the operation in vCPU's
thread.

As i mentioned before the memory region used for vNVDIMM
flush interface should be MMIO and consider its support
on other hypervisors, so we do better push this async
mechanism into the flush interface design itself rather
than depends on kvm async-page-fault.