Documentation/protection-keys: add AArch64 to documentation

As POE support was recently added, update the documentation.

Also note that kernel threads have a default protection key register value.
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241001133618.1547996-3-joey.gouly@arm.com
[will: Adjusted wording based on feedback from Kevin]
Signed-off-by: Will Deacon <will@kernel.org>

Documentation/core-api/protection-keys.rst

@@ -12,7 +12,10 @@ Pkeys Userspace (PKU) is a feature which can be found on:
         * Intel server CPUs, Skylake and later
         * Intel client CPUs, Tiger Lake (11th Gen Core) and later
         * Future AMD CPUs
+        * arm64 CPUs implementing the Permission Overlay Extension (FEAT_S1POE)
 
+x86_64
+======
 Pkeys work by dedicating 4 previously Reserved bits in each page table entry to
 a "protection key", giving 16 possible keys.
 
@@ -28,6 +31,22 @@ register.  The feature is only available in 64-bit mode, even though there is
 theoretically space in the PAE PTEs.  These permissions are enforced on data
 access only and have no effect on instruction fetches.
 
+arm64
+=====
+
+Pkeys use 3 bits in each page table entry, to encode a "protection key index",
+giving 8 possible keys.
+
+Protections for each key are defined with a per-CPU user-writable system
+register (POR_EL0).  This is a 64-bit register encoding read, write and execute
+overlay permissions for each protection key index.
+
+Being a CPU register, POR_EL0 is inherently thread-local, potentially giving
+each thread a different set of protections from every other thread.
+
+Unlike x86_64, the protection key permissions also apply to instruction
+fetches.
+
 Syscalls
 ========
 
@@ -38,11 +57,10 @@ There are 3 system calls which directly interact with pkeys::
 	int pkey_mprotect(unsigned long start, size_t len,
 			  unsigned long prot, int pkey);
 
-Before a pkey can be used, it must first be allocated with
-pkey_alloc().  An application calls the WRPKRU instruction
-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().
+Before a pkey can be used, it must first be allocated with pkey_alloc().  An
+application writes to the architecture specific CPU register directly in order
+to change access permissions to memory covered with a key.  In this example
+this is wrapped by a C function called pkey_set().
 ::
 
 	int real_prot = PROT_READ|PROT_WRITE;
@@ -64,9 +82,9 @@ 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 around writing to the CPU register.
+          Example implementations can be found in
+          tools/testing/selftests/mm/pkey-{arm64,powerpc,x86}.h
 
 Behavior
 ========
@@ -96,3 +114,7 @@ with a read()::
 The kernel will send a SIGSEGV in both cases, but si_code will be set
 to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
 the plain mprotect() permissions are violated.
+
+Note that kernel accesses from a kthread (such as io_uring) will use a default
+value for the protection key register and so will not be consistent with
+userspace's value of the register or mprotect().