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Subject: Re: [Qemu-devel] [RFC] qemu: Add virtio pmem device
To:     Stefan Hajnoczi <stefanha@gmail.com>,
        Pankaj Gupta <pagupta@redhat.com>
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        mst@redhat.com, ross zwisler <ross.zwisler@intel.com>,
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        hch@infradead.org, pbonzini@redhat.com, stefanha@redhat.com,
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        imammedo@redhat.com, dan j williams <dan.j.williams@intel.com>,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <36218ea8-5fa8-e670-104d-616da790a333@redhat.com>
Date:   Mon, 9 Apr 2018 08:42:35 +0200
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On 09.04.2018 05:26, Stefan Hajnoczi wrote:
> On Thu, Apr 05, 2018 at 08:09:26AM -0400, Pankaj Gupta wrote:
>>> Will this raw file already have the "disk information header" (no idea
>>> how that stuff is called) encoded? Are there any plans/possible ways to
>>>
>>> a) automatically create the headers? (if that's even possible)
>>
>> Its raw. Right now we are just supporting raw format.  
>>
>> As this is direct mapping of memory into guest address space, I don't
>> think we can have an abstraction of headers for block specific features.
>> Or may be we can get opinion of others(Qemu block people) it is at all possible?
> 
> memdev and the block layer are completely separate.  The block layer
> isn't designed for memory-mapped access.
> 

Not questioning if this is the right thing to do now. I was wondering if
we could expose any block device in the future as virtio-pmem. And I
think with quite some work it could be possible.

As you said, we will need some buffering. Maybe userfaultfd and friends
(WP) could allow to implement that.

> I think it makes sense to use memdev here.  If the user wants a block
> device, they should use an emulated block device, not virtio-pmem,
> because buffering is necessary anyway when an image file format is used.
> 
> Stefan
> 


-- 

Thanks,

David / dhildenb