linux-kernel - Re: [PATCH 0/5] Add vhost-blk support

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Date:	Tue, 17 Jul 2012 15:02:45 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Stefan Hajnoczi <stefanha@...il.com>, Asias He <asias@...hat.com>,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	Benjamin LaHaise <bcrl@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] Add vhost-blk support

Il 17/07/2012 14:48, Michael S. Tsirkin ha scritto:
> On Tue, Jul 17, 2012 at 01:03:39PM +0100, Stefan Hajnoczi wrote:
>> On Tue, Jul 17, 2012 at 12:54 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
>>>> Knowing the answer to that is important before anyone can say whether
>>>> this approach is good or not.
>>>>
>>>> Stefan
>>>
>>> Why is it?
>>
>> Because there might be a fix to kvmtool which closes the gap.  It
>> would be embarassing if vhost-blk was pushed just because no one
>> looked into what is actually going on.
> 
> Embarrasing to whom? Is someone working on an optimization that
> makes the work in question redundant, with posting just around
> the corner? Then maybe the thing to do is just wait a bit.

Of course there is work going on to make QEMU perform better.  Not sure
about lkvm.

>> And on the flipside, hard evidence of an overhead that cannot be
>> resolved could be good reason to do more vhost devices in the future.
> 
> How can one have hard evidence of an overhead that cannot be resolved?

Since we do have two completely independent userspaces (lkvm and
data-plane QEMU), you can build up some compelling evidence of an
overhead that cannot be resolved in user space.

>> Either way, it's useful to do this before going further.
> 
> I think each work should be discussed on its own merits.  Maybe
> vhost-blk is just well written. So? What is your conclusion?

If it's just that vhost-blk is written well, my conclusion is that lkvm
people should look into improving their virtio-blk userspace.  We take
hints from each other all the time, for example virtio-scsi will have
unlocked kick in 3.6.

Why can't vhost-* just get into staging, and we call it a day?

Paolo
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