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

From: Michael S. Tsirkin
Date: Tue Jul 17 2012 - 09:26:12 EST

On Tue, Jul 17, 2012 at 03:02:45PM +0200, 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.
> >>>>
> >>>> 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.

OK, so what you are saying benchmark against data-plane QEMU?
I agree actually. Asias, any data?

> >> 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

staging is not a destination. Even if we put a driver in staging it
won't stay there indefinitely if qemu-kvm is not using it,
something that doesn't seem to be addressed for vhost-blk yet.

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