linux-kernel - [PATCH v5 3/4] Documentation/x86: Add the AMX enabling example

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Message-Id: <20220922195810.23248-4-chang.seok.bae@intel.com>
Date:   Thu, 22 Sep 2022 12:58:09 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com
Cc:     hpa@...or.com, corbet@....net, bagasdotme@...il.com,
        tony.luck@...el.com, yang.zhong@...el.com,
        linux-doc@...r.kernel.org, linux-man@...r.kernel.org,
        linux-kernel@...r.kernel.org, chang.seok.bae@...el.com
Subject: [PATCH v5 3/4] Documentation/x86: Add the AMX enabling example

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

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Reviewed-by: Thiago Macieira <thiago.macieira@...el.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-doc@...r.kernel.org
---
Changes from v1:
* Update the description without mentioning CPUID & XGETBV (Dave Hansen).

Changes from v2:
* Massage sentences (Bagas Sanjaya).
* Adjust the example with the (future) prctl.h.
---
 Documentation/x86/xstate.rst | 55 ++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst
index e954e79af4ce..23b1c9f3efb2 100644
--- a/Documentation/x86/xstate.rst
+++ b/Documentation/x86/xstate.rst
@@ -80,6 +80,61 @@ 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
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Below is the example of how userspace applications enable
+TILE_DATA dynamically:
+
+  1. The application first needs to query the kernel for AMX
+     support::
+
+        #include <asm/prctl.h>
+        #include <sys/syscall.h>
+        #include <stdio.h>
+        #include <unistd.h>
+
+        #ifndef ARCH_GET_XCOMP_SUPP
+        #define ARCH_GET_XCOMP_SUPP  0x1021
+        #endif
+
+        #ifndef ARCH_XCOMP_TILECFG
+        #define ARCH_XCOMP_TILECFG   17
+        #endif
+
+        #ifndef ARCH_XCOMP_TILEDATA
+        #define ARCH_XCOMP_TILEDATA  18
+        #endif
+
+        #define MASK_XCOMP_TILE      ((1 << ARCH_XCOMP_TILECFG) | \
+                                      (1 << ARCH_XCOMP_TILEDATA))
+
+        unsigned long features;
+        long rc;
+
+        ...
+
+        rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
+
+        if (!rc && (features & MASK_XCOMP_TILE) == MASK_XCOMP_TILE)
+            printf("AMX is available.\n");
+
+  2. After that, determining support for AMX, an application must
+     explicitly ask permission to use it::
+
+        #ifndef ARCH_REQ_XCOMP_PERM
+        #define ARCH_REQ_XCOMP_PERM  0x1023
+        #endif
+
+        ...
+
+        rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, ARCH_XCOMP_TILEDATA);
+
+        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