[PATCH 1/2] Documentation/x86: Add the AMX enabling example

["Chang S. Bae" <chang.seok.bae@intel.com>]

Explain steps to enable the dynamic feature with a code example.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 Documentation/x86/xstate.rst | 48 ++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst
index 5cec7fb558d6..9597e6caa30e 100644
--- a/Documentation/x86/xstate.rst
+++ b/Documentation/x86/xstate.rst
@@ -64,6 +64,54 @@ the handler allocates a larger xstate buffer for the task so the large
 state can be context switched. In the unlikely cases that the allocation
 fails, the kernel sends SIGSEGV.
 
+AMX TILE_DATA enabling example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The following steps dynamically enable TILE_DATA:
+
+  1. **Check the feature availability**. AMX_TILE is enumerated in CPUID
+     leaf 7, sub-leaf 0, bit 24 of EDX. If available, ``/proc/cpuinfo``
+     shows ``amx_tile`` in the flag entry of the CPUs.  Given that, the
+     kernel may have set XSTATE component 18 in the XCR0 register. But a
+     user needs to ensure the kernel support via the ARCH_GET_XCOMP_SUPP
+     option::
+
+        #include <asm/prctl.h>
+        #include <sys/syscall.h>
+	#include <stdio.h>
+        #include <unistd.h>
+
+        #define ARCH_GET_XCOMP_SUPP  0x1021
+
+        #define XFEATURE_XTILECFG    17
+        #define XFEATURE_XTILEDATA   18
+        #define XFEATURE_MASK_XTILE ((1 << XFEATURE_XTILECFG) | (1 << XFEATURE_XFILEDATA))
+
+        unsigned long features;
+        long rc;
+
+        ...
+
+        rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
+
+        if (!rc && features & XFEATURE_MASK_XTILE == XFEATURE_MASK_XTILE)
+            printf("AMX is available.\n");
+
+  2. **Request permission**. Now it is found that the kernel supports the
+     feature. But the permission is not automatically given. A user needs
+     to explicitly request it via the ARCH_REQ_XCOMP_PERM option::
+
+        #define ARCH_REQ_XCOMP_PERM  0x1023
+
+        ...
+
+        rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA);
+
+        if (!rc)
+            printf("AMX is ready for use.\n");
+
+Note this example does not include the sigaltstack preparation.
+
 Dynamic features in signal frames
 ---------------------------------
 
-- 
2.17.1