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

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Date:	Tue, 17 Jul 2012 12:56:31 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Asias He <asias@...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, nab@...ux-iscsi.org,
	target-devel@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [PATCH 0/5] Add vhost-blk support

Il 17/07/2012 12:49, Michael S. Tsirkin ha scritto:
>> Ok, that would make more sense.  One difference between vhost-blk and
>> vhost-net is that for vhost-blk there are also management actions that
>> would trigger the switch, for example a live snapshot.
>> So a prerequisite for vhost-blk would be that it is possible to disable
>> it on the fly while the VM is running, as soon as all in-flight I/O is
>> completed.
> 
> It applies for vhost-net too. For example if you bring link down,
> we switch to userspace. So vhost-net supports this switch on the fly.

Cool.

>> (Note that, however, this is not possible for vhost-scsi, because it
>> really exposes different hardware to the guest.  It must not happen that
>> a kernel upgrade or downgrade toggles between userspace SCSI and
>> vhost-scsi, for example).
> 
> I would say this is not a prerequisite for merging in qemu.
> It might be a required feature for production but it
> is also solvable at the management level.

I'm thinking of the level interrupts here.  You cannot make a change in
the guest, and have it do completely unrelated changes the hardware that
the guest sees.

>>>> having to
>>>> support the API; having to handle transition from one more thing when
>>>> something better comes out.
>>>
>>> Well this is true for any code. If the limited featureset which
>>> vhost-blk can accelerate is something many people use, then accelerating
>>> by 5-15% might outweight support costs.
>>
>> It is definitely what people use if they are interested in performance.
> 
> In that case it seems to me we should stop using the feature set as
> an argument and focus on whether the extra code is worth the 5-15% gain.
> No one seems to have commented on that so everyone on list thinks that
> aspect is OK?

I would like to see a breakdown of _where_ the 5-15% lies, something
like http://www.linux-kvm.org/page/Virtio/Block/Latency.

> Kernel merge windows is coming up and I would like to see whether
> any of vhost-blk / vhost-scsi is going to be actually used by userspace.
> I guess we could tag it for staging but would be nice to avoid that.

Staging would be fine by me for both vhost-blk and vhost-scsi.

Paolo
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