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[Xen-devel] Re: [RFC] [PATCH] sysfs support for Xen attributes

Greg KH wrote:

Why is xen special from the rest of the kernel in regards to adding
files to sysfs?  What does your infrastructure add that is not currently
already present for everyone to use today?

I think it comes down to simplification for non-driver code, which is admittedly not the mainstream use model for sysfs.

Why is the xen version any different from any other module or driver
version in the kernel? (hint, use the interface that is availble for
this already...)

The module version? Xen is not a module nor a driver, so that interface doesn't quite serve the purpose. True, one could create a "Xen module" that talks to Xen the hypervisor, but then the version interface would provide the version of the xen module, not the version of the xen hypervisor. /sys/xen/version may not be the best example for this discussion. What is important is that this attribute is obtained from Xen using a hypercall. Sysfs works great to prove the xen version and other similar xen attributes to userspace.

You have access to the current tree as well as we do to be able to
answer this question :)

Right. Dumb question.

You don't have to create a driver subsystem to be able to add stuff to
sysfs, what makes you think that?

Sorry, you are right. But you do need to have s struct dev or use kobjects. What I want is an interface to create sysfs files using a path as a parameter, rather than a struct kobject.

did you look at debugfs?



no. configfs may be a better choice. I would still want a higher-level kernel interface similar to what is in the patch, as explained below. But I think sysfs may be more appropriate because attributes show up automatically without a user-space action being taken.

What is wrong with the current kobject/sysfs/driver model interface that
made you want to create this extra code?

Nothing is wrong, but I want a higher-level interface, to be able to create files and directories using a path, and to allow a code that is not associated with a device to create sysfs files by specifying a path. e.g., create(path, mode, ...).

Currently in xeno-linux there are several files under /proc/xen. These are created by different areas of the xeno-linux kernel. In xeno-linux today there is a single higher-level routine that each of these different areas uses to create its own file under /proc/xen. In other words, I think there should be a unifying element to the interface because the callers are not organized within a single module.

Aren't you already going to have a xen virtual bus in sysfs and the
driver model?  Why not just put your needed attributes there, where they
belong (on the devices themselves)?

the xenbus, which is now in xen 3.0, allows kernels running in xen domains to get access to virtual devices hosted in a driver domain/domain0. But the attributes I am creating in /sys/xen are xen attributes, not device attributes. The difference is important to consumers of the attributes. I could create a device just to export hypervisor attributes, but I think the what I've done is simpler.

+#define __sysfs_ref__


A simple way to denote functions that get a reference to a reference counted object. e.g., int __sysfs_ref__ foo(void); gone.

+struct xen_sysfs_object;
+struct xen_sysfs_attr {
+       struct bin_attribute attr;
+       ssize_t (*show)(void *, char *) ;
+       ssize_t (*store)(void *, const char *, size_t) ;
+       ssize_t (*read)(void *, char *, loff_t, size_t );
+       ssize_t (*write)(void *, char *, loff_t, size_t) ;

Why a binary attribute?  Do you want to have more than one single piece
of info in here?  If so, no.

To facilitate creation of binary files. struct bin_attribute contains a struct attribute, so it is an alternative to using a union.

Mike (hoping he doesn't end up on linux kernel monkey log)


Mike D. Day
STSM and Architect, Open Virtualization
IBM Linux Technology Center

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