linux-kernel - Re: [PATCH v7 16/46] virtio

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Message-ID: <20141201130258.6f01c1e3.cornelia.huck@de.ibm.com>
Date:	Mon, 1 Dec 2014 13:02:58 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	rusty@....ibm.com, nab@...ux-iscsi.org, pbonzini@...hat.com,
	thuth@...ux.vnet.ibm.com, Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v7 16/46] virtio_blk: v1.0 support

On Mon, 1 Dec 2014 13:46:45 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> On Mon, Dec 01, 2014 at 12:33:15PM +0100, Cornelia Huck wrote:
> > On Mon, 1 Dec 2014 11:26:58 +0200
> > "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > 
> > > For some places on data path, it might be worth it
> > > to cache the correct value e.g. as part of device
> > > structure. This replaces a branch with a memory load,
> > > so the gain would have to be measured, best done
> > > separately?
> > 
> > I think we'll want to do some measuring once the basic structure is
> > in place anyway.
> 
> What's meant by in place here?

That this patchset is ready :)

> 
> > We should make sure that e.g. s390 only takes minor
> > hit due to all that swapping that is needed for standard-compliant
> > devices. Caching the value might certainly help in some paths.
> 
> Well, this is queued in linux-next for 3.19, so
> now's the time to do it :)

So much to do, so little time...

I'm still feeling a bit uncomfortable with some of the changes
(virtio-scsi etc.) as I have not been able to test them yet (as there's
no converted qemu for these yet). The virtio-net and virtio-blk changes
seem sane, though, and virtio-ccw should be fine as well.

OTOH, it's not like we're introducing new external interfaces, so later
rework should be fine.

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