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Date: Wed, 16 Mar 2011 15:09:58 +0100
From: Christoph Hellwig <hch@lst.de>
Message-ID: <20110316140958.GB21877@lst.de>
References: <20110315141049.GA30627@lst.de> <20110315141644.GA30803@lst.de>
	<87y64fhfjw.fsf@rustcorp.com.au>
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Subject: [Qemu-devel] Re: [PATCH, RFC] virtio_blk: add cache control support
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To: Rusty Russell <rusty@rustcorp.com.au>
Cc: kwolf@redhat.com, aliguori@us.ibm.com, stefanha@gmail.com, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, prerna@linux.vnet.ibm.com, Christoph Hellwig <hch@lst.de>

On Wed, Mar 16, 2011 at 02:39:39PM +1030, Rusty Russell wrote:
> > +	if (strncmp(buf, "write through", sizeof("write through") - 1) == 0) {
> > +		;
> > +	} else if (strncmp(buf, "write back", sizeof("write back") - 1) == 0) {
> 
>    Is there a reason we're not letting gcc and/or strcmp do the
> optimization work here?

I'm happ to switch strcmp.

> > +	vdev->config->set(vdev, offsetof(struct virtio_blk_config, features),
> > +	                  &features, sizeof(features));
> > +
> > +	vdev->config->get(vdev, offsetof(struct virtio_blk_config, features),
> > +			  &features2, sizeof(features2));
> > +
> > +	if ((features & VIRTIO_BLK_RT_WCE) !=
> > +	    (features2 & VIRTIO_BLK_RT_WCE))
> > +		return -EIO;
> 
>    This seems like a debugging check you left in.  Or do you suspect
> some issues?

No, it's intentional.  config space writes can't return errors, so we need
to check that the value has really changed.  I'll add a comment explaining it.