docs: move x86 documentation into Documentation/arch/

Move the x86 documentation under Documentation/arch/ as a way of cleaning
up the top-level directory and making the structure of our docs more
closely match the structure of the source directories it describes.

All in-kernel references to the old paths have been updated.

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-arch@vger.kernel.org
Cc: x86@kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20230315211523.108836-1-corbet@lwn.net/
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 74 insertions, 0 deletions

diff --git a/Documentation/arch/x86/xstate.rst b/Documentation/arch/x86/xstate.rst
new file mode 100644
index 000000000000..5cec7fb558d6
--- /dev/null
+++ b/Documentation/arch/x86/xstate.rst
@@ -0,0 +1,74 @@
+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.
+
+Dynamic features in signal frames
+---------------------------------
+
+Dynamcally enabled features are not written to the signal frame upon signal
+entry if the feature is in its initial configuration.  This differs from
+non-dynamic features which are always written regardless of their
+configuration.  Signal handlers can examine the XSAVE buffer's XSTATE_BV
+field to determine if a features was written.