Re: [Qemu-devel] [PATCH 2/4] block: add a helper to change writeback mode on the fly

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Mar 15, 2011 at 03:11:32PM +0100, Christoph Hellwig wrote:
> Add a new bdrv_change_cache that can set/clear the writeback flag
> at runtime by stopping all I/O and closing/reopening the image file.
> 
> All code is based on a patch from Prerna Saxena <prerna@linux.vnet.ibm.com>
> with minimal refactoring.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
>  
> +static int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
> +{
> +    BlockDriver *drv = bs->drv;
> +    int ret;
> +
> +    if (bdrv_flags == bs->open_flags) {
> +        return 0;
> +    }
> +
> +    /* Quiesce IO for the given block device */
> +    qemu_aio_flush();
> +    bdrv_flush(bs);
> +
> +    bdrv_close(bs);
> +    ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
> +
> +    /*
> +     * A failed attempt to reopen the image file must lead to 'abort()'
> +     */
> +    if (ret != 0) {
> +        abort();
> +    }
> +
> +    return ret;
> +}



> +
> +int bdrv_change_cache(BlockDriverState *bs, bool enable)
> +{
> +    int bdrv_flags = 0;
> +
> +    bdrv_flags = bs->open_flags & ~BDRV_O_CACHE_WB;
> +    if (enable) {
> +        bdrv_flags |= BDRV_O_CACHE_WB;
> +    }
> +
> +    return bdrv_reopen(bs, bdrv_flags);
> +}

Is there any way we can manage todo this *without* closing and
re-opening the file descriptor ?

One of the things we're considering for the future is to enable
QEMU to be passed open FD(s) for its drives, from the management
system, instead of having QEMU open the files itself. This will
make it possible to have strong, per-VM selinux isolation for
disks stored on NFS. It will also make it easier to let us run
QEMU inside a private filesystem namespace (CLONE_NEWNS) and not
have to expose any files in that namespace except the QEMU binary
& a few device nodes (particularly useful for hotplug since we
won't know what files need to be visible inside the private namespace
ahead of time).

If QEMU expects to close+reopen any of its disks at any time the
guest requests, this will complicate life somewhat

Regards,
Daniel
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