linux-kernel - Re: [PATCH, RFC] virtio_blk: add cache control support

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Message-ID: <87y64fhfjw.fsf@rustcorp.com.au>
Date:	Wed, 16 Mar 2011 14:39:39 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Hellwig <hch@....de>, qemu-devel@...gnu.org,
	linux-kernel@...r.kernel.org
Cc:	stefanha@...il.com, kwolf@...hat.com, prerna@...ux.vnet.ibm.com,
	aliguori@...ibm.com
Subject: Re: [PATCH, RFC] virtio_blk: add cache control support

On Tue, 15 Mar 2011 15:16:44 +0100, Christoph Hellwig <hch@....de> wrote:
> Add support for the new dynamic features config space field to allow
> en/disabling the write cache at runtime.  The userspace interface is
> a SCSI-compatible sysfs attribute.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Hi Christoph,

   Neat work.  Minor comments:

> +	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?

> +	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?

Thanks,
Rusty.
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