f_ops flag to speed up compatible ioctls in linux kernel

[Michael S. Tsirkin]

Hello!
Currently, on the x86_64 architecture, its quite tricky to make
a char device ioctl work for an x86 executables.
In particular,
   1. there is a requirement that ioctl number is unique -
      which is hard to guarantee especially for out of kernel modules
   2. there's a performance huge overhead for each compat call - there's
      a hash lookup in a global hash inside a lock_kernel -
      and I think compat performance *is* important.

Further, adding a command to the ioctl suddenly requires changing
two places - registration code and ioctl itself.
  
However, if all ioctl commands for a specific device
are not passing around pointers and long longs,
(relatively common case), and sometimes even
if they are, this ioctl is 64/32 bit compatible -
so why isnt there a simple way to declare this fact?

Here's a patch that simply adds a flag field to f_ops
that will cause all ioctls to get forwarded directly to the
64 bit call.

If all ioctls are compatible for a character device, a flag just
has to be set there, before the device is registered.

MST

diff -ruw linux-2.6.8.1/fs/compat.c linux-2.6.8.1-built/fs/compat.c
--- linux-2.6.8.1/fs/compat.c	2004-08-14 13:55:31.000000000 +0300
+++ linux-2.6.8.1-built/fs/compat.c	2004-09-01 09:52:10.126944256 +0300
@@ -392,7 +392,8 @@
 	if(!filp)
 		goto out2;
 
-	if (!filp->f_op || !filp->f_op->ioctl) {
+	if (!filp->f_op || !filp->f_op->ioctl ||
+          (filp->f_op->fops_flags & FOPS_IOCTL_COMPAT)) {
 		error = sys_ioctl (fd, cmd, arg);
 		goto out;
 	}
diff -ruw linux-2.6.8.1/include/linux/fs.h linux-2.6.8.1-built/include/linux/fs.h
--- linux-2.6.8.1/include/linux/fs.h	2004-08-14 13:55:09.000000000 +0300
+++ linux-2.6.8.1-built/include/linux/fs.h	2004-09-01 09:50:07.265622016 +0300
@@ -871,6 +871,7 @@
  */
 struct file_operations {
 	struct module *owner;
+	unsigned long fops_flags;		/* Flags, listed below. */
 	loff_t (*llseek) (struct file *, loff_t, int);
 	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
 	ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
@@ -896,6 +897,8 @@
 	int (*dir_notify)(struct file *filp, unsigned long arg);
 };
 
+#define FOPS_IOCTL_COMPAT 0x00000001 /* ioctl is 32/64 bit compatible */
+
 struct inode_operations {
 	int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
 	struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
-
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