[tip: x86/fpu] Documentation/x86: Add documentation for using dynamic XSTATE features

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     d7a9590f608dbedd917eb0857a074accdf0d3919
Gitweb:        https://git.kernel.org/tip/d7a9590f608dbedd917eb0857a074accdf0d3919
Author:        Chang S. Bae <chang.seok.bae@xxxxxxxxx>
AuthorDate:    Tue, 26 Oct 2021 02:11:57 -07:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Thu, 28 Oct 2021 14:54:58 +02:00

Documentation/x86: Add documentation for using dynamic XSTATE features

Explain how dynamic XSTATE features can be enabled via the
architecture-specific prctl() along with dynamic sigframe size and
first use trap handling.

Fix:

Documentation/x86/xstate.rst:15: WARNING: Title underline too short.

as reported by Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>

Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20211026091157.16711-1-chang.seok.bae@xxxxxxxxx
---
 Documentation/x86/index.rst  |  1 +-
 Documentation/x86/xstate.rst | 65 +++++++++++++++++++++++++++++++++++-
 2 files changed, 66 insertions(+)
 create mode 100644 Documentation/x86/xstate.rst

diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index 3830483..f498f1d 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -37,3 +37,4 @@ x86-specific Documentation
    sgx
    features
    elf_auxvec
+   xstate
diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst
new file mode 100644
index 0000000..65de3f0
--- /dev/null
+++ b/Documentation/x86/xstate.rst
@@ -0,0 +1,65 @@
+Using XSTATE features in user space applications
+================================================
+
+The x86 architecture supports floating-point extensions which are
+enumerated via CPUID. Applications consult CPUID and use XGETBV to
+evaluate which features have been enabled by the kernel XCR0.
+
+Up to AVX-512 and PKRU states, these features are automatically enabled by
+the kernel if available. Features like AMX TILE_DATA (XSTATE component 18)
+are enabled by XCR0 as well, but the first use of related instruction is
+trapped by the kernel because by default the required large XSTATE buffers
+are not allocated automatically.
+
+Using dynamically enabled XSTATE features in user space applications
+--------------------------------------------------------------------
+
+The kernel provides an arch_prctl(2) based mechanism for applications to
+request the usage of such features. The arch_prctl(2) options related to
+this are:
+
+-ARCH_GET_XCOMP_SUPP
+
+ arch_prctl(ARCH_GET_XCOMP_SUPP, &features);
+
+ ARCH_GET_XCOMP_SUPP stores the supported features in userspace storage of
+ type uint64_t. The second argument is a pointer to that storage.
+
+-ARCH_GET_XCOMP_PERM
+
+ arch_prctl(ARCH_GET_XCOMP_PERM, &features);
+
+ ARCH_GET_XCOMP_PERM stores the features for which the userspace process
+ has permission in userspace storage of type uint64_t. The second argument
+ is a pointer to that storage.
+
+-ARCH_REQ_XCOMP_PERM
+
+ arch_prctl(ARCH_REQ_XCOMP_PERM, feature_nr);
+
+ ARCH_REQ_XCOMP_PERM allows to request permission for a dynamically enabled
+ feature or a feature set. A feature set can be mapped to a facility, e.g.
+ AMX, and can require one or more XSTATE components to be enabled.
+
+ The feature argument is the number of the highest XSTATE component which
+ is required for a facility to work.
+
+When requesting permission for a feature, the kernel checks the
+availability. The kernel ensures that sigaltstacks in the process's tasks
+are large enough to accommodate the resulting large signal frame. It
+enforces this both during ARCH_REQ_XCOMP_SUPP and during any subsequent
+sigaltstack(2) calls. If an installed sigaltstack is smaller than the
+resulting sigframe size, ARCH_REQ_XCOMP_SUPP results in -ENOSUPP. Also,
+sigaltstack(2) results in -ENOMEM if the requested altstack is too small
+for the permitted features.
+
+Permission, when granted, is valid per process. Permissions are inherited
+on fork(2) and cleared on exec(3).
+
+The first use of an instruction related to a dynamically enabled feature is
+trapped by the kernel. The trap handler checks whether the process has
+permission to use the feature. If the process has no permission then the
+kernel sends SIGILL to the application. If the process has permission then
+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.