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

From: Asias He
Date: Wed Jul 18 2012 - 04:45:36 EST

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@xxxxxxxxxx> wrote:
Knowing the answer to that is important before anyone can say whether
this approach is good or not.


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.


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