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

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Message-ID: <5006781F.1060900@redhat.com>
Date:	Wed, 18 Jul 2012 16:47:27 +0800
From:	Asias He <asias@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	"Michael S. Tsirkin" <mst@...hat.com>,
	Stefan Hajnoczi <stefanha@...il.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

On 07/17/2012 09:02 PM, Paolo Bonzini wrote:
> 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.

Of course for lkvm also.

>>> 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.

This does not build the hard evidence either. How can one prove that 
userspace lkvm and data-plane QEMU can not be improved further? The same 
for vhost-blk.

>>> 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?

OK. I'm fine with staging.

-- 
Asias


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