Explain steps to enable the dynamic feature with a code example.
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Thiago Macieira <thiago.macieira@xxxxxxxxx>
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
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
