Documentation: x86: convert protection-keys.txt to reST

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.

Signed-off-by: Changbin Du <[e-mail]>
Reviewed-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/x86/index.rst

···
    tlb
    mtrr
    pat
+   protection-keys

Documentation/x86/protection-keys.txt Documentation/x86/protection-keys.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+======================
+Memory Protection Keys
+======================
+
 Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature
 which is found on Intel's Skylake "Scalable Processor" Server CPUs.
 It will be avalable in future non-server parts.
···
 permissions are enforced on data access only and have no effect on
 instruction fetches.
 
-=========================== Syscalls ===========================
+Syscalls
+========
 
-There are 3 system calls which directly interact with pkeys:
+There are 3 system calls which directly interact with pkeys::
 
 	int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
 	int pkey_free(int pkey);
···
 directly in order to change access permissions to memory covered
 with a key.  In this example WRPKRU is wrapped by a C function
 called pkey_set().
+::
 
 	int real_prot = PROT_READ|PROT_WRITE;
 	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE);
···
 	... application runs here
 
 Now, if the application needs to update the data at 'ptr', it can
-gain access, do the update, then remove its write access:
+gain access, do the update, then remove its write access::
 
 	pkey_set(pkey, 0); // clear PKEY_DISABLE_WRITE
 	*ptr = foo; // assign something
 	pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_WRITE again
 
 Now when it frees the memory, it will also free the pkey since it
-is no longer in use:
+is no longer in use::
 
 	munmap(ptr, PAGE_SIZE);
 	pkey_free(pkey);
 
-(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
- An example implementation can be found in
- tools/testing/selftests/x86/protection_keys.c)
+.. note:: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
+          An example implementation can be found in
+          tools/testing/selftests/x86/protection_keys.c.
 
-=========================== Behavior ===========================
+Behavior
+========
 
 The kernel attempts to make protection keys consistent with the
-behavior of a plain mprotect().  For instance if you do this:
+behavior of a plain mprotect().  For instance if you do this::
 
 	mprotect(ptr, size, PROT_NONE);
 	something(ptr);
 
-you can expect the same effects with protection keys when doing this:
+you can expect the same effects with protection keys when doing this::
 
 	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ);
 	pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey);
 	something(ptr);
 
 That should be true whether something() is a direct access to 'ptr'
-like:
+like::
 
 	*ptr = foo;
 
 or when the kernel does the access on the application's behalf like
-with a read():
+with a read()::
 
 	read(fd, ptr, 1);