Re: [Xen-devel] [PATCH 5/6] libxl: introduce libxl__alloc_vdev

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 30 Mar 2012, Ian Campbell wrote:
> On Fri, 2012-03-30 at 12:43 +0100, Stefano Stabellini wrote:
> > On Tue, 27 Mar 2012, Ian Campbell wrote:
> > > > +static char * libxl__alloc_vdev(libxl__gc *gc, uint32_t domid, 
> > > > xs_transaction_t t,
> > > > +        libxl_device_disk *disk)
> > > > +{
> > > > +    int rc = 0;
> > > > +    libxl_device_disk *disks = NULL;
> > > > +    int num = 0, idx = -1, max_idx = -1, i = 0;
> > > > +
> > > > +    rc = libxl__append_disk_list_of_type(gc, domid, t, "vbd", &disks, 
> > > > &num);
> > > > +    if (rc) goto out;
> > > > +
> > > > +    rc = libxl__append_disk_list_of_type(gc, domid, t, "tap", &disks, 
> > > > &num);
> > > > +    if (rc) goto out;
> > > > +
> > > > +    rc = libxl__append_disk_list_of_type(gc, domid, t, "qdisk", 
> > > > &disks, &num);
> > > > +    if (rc) goto out;
> > > 
> > > This is basically an open-coded version of libxl_disk_list, isn't it?
> > 
> > yes it is similar, but as you can see the "smart" bit is in
> > libxl__append_disk_list_of_type anyway
> 
> It's also in the list "vbd", "tap", "qdisk" which now need to be sync'd
> in more than one place if it changes.
> 
> > > For this use though wouldn't it be as easy to simply iterate over idx
> > > checking if a frontend dir exists?
> > 
> > We could try one at a time: 
> > 
> > xvd(a + idx) -> dev_number -> xs_read * 3
> > 
> > this could work, even though we would still need to read 3 possible
> > paths (tap, qdisk, vbd) for each one.
> 
> You can check for the existence of the frontend dir here. I think this
> is safe since only the content of the dir is(/should be) writeable by
> the guest, the existence ofd that node is completely controlled by the
> toolstack.

I like it, I'll do that

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