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

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Message-ID: <20141201092658.GC15607@redhat.com>
Date:	Mon, 1 Dec 2014 11:26:58 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	cornelia.huck@...ibm.com, 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, Dec 01, 2014 at 09:16:41AM +0100, David Hildenbrand wrote:
> > Based on patch by Cornelia Huck.
> > 
> > Note: for consistency, and to avoid sparse errors,
> >       convert all fields, even those no longer in use
> >       for virtio v1.0.
> > 
> > Signed-off-by: Cornelia Huck <cornelia.huck@...ibm.com>
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ...
> > 
> > -static unsigned int features[] = {
> > +static unsigned int features_legacy[] = {
> >  	VIRTIO_BLK_F_SEG_MAX, VIRTIO_BLK_F_SIZE_MAX, VIRTIO_BLK_F_GEOMETRY,
> >  	VIRTIO_BLK_F_RO, VIRTIO_BLK_F_BLK_SIZE, VIRTIO_BLK_F_SCSI,
> >  	VIRTIO_BLK_F_WCE, VIRTIO_BLK_F_TOPOLOGY, VIRTIO_BLK_F_CONFIG_WCE,
> >  	VIRTIO_BLK_F_MQ,
> > +}
> > +;
> > +static unsigned int features[] = {
> > +	VIRTIO_BLK_F_SEG_MAX, VIRTIO_BLK_F_SIZE_MAX, VIRTIO_BLK_F_GEOMETRY,
> > +	VIRTIO_BLK_F_RO, VIRTIO_BLK_F_BLK_SIZE,
> > +	VIRTIO_BLK_F_TOPOLOGY,
> > +	VIRTIO_BLK_F_MQ,
> > +	VIRTIO_F_VERSION_1,
> 
> We can fit this into less lines, like done for features_legacy.
> 
> I was asking myself if we could do the conversion of the statical values
> somehow upfront, to reduce the patch size and avoid cpu_to_virtio.* at those
> places.
> 
> Otherwise looks good to me.
> 

I don't see how we can reduce the patch size.
For BE architectures it's dynamic, so at best the values
will become macros/incline functions taking a flag.

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?

-- 
MST
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