[PATCH 01/10] x86/fpu: Document signal frame portability

[Andrei Vagin <avagin@google.com>]

The x86 signal frame is designed to be self-describing, with the
'xstate_size' field in the software-reserved bytes indicating the actual
size of the context. This design is required for portability, allowing a
signal frame created on a system with a specific set of xstate features
to be restored on a machine with a different (larger) set of features.

Document this contract in the uabi headers and Documentation/. This
requirement is critical for checkpoint/restore tools like CRIU, which
should be able to migrate processes across machines with heterogeneous
FPU capabilities. Note that portability is generally limited to CPUs
from the same vendor (e.g., Intel vs. AMD) due to potential differences
in xstate layouts.

Signed-off-by: Andrei Vagin <avagin@google.com>
---
 Documentation/arch/x86/xstate.rst      | 13 +++++++++++++
 arch/x86/include/uapi/asm/sigcontext.h | 13 +++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/Documentation/arch/x86/xstate.rst b/Documentation/arch/x86/xstate.rst
index cec05ac464c1..facd97b26bf1 100644
--- a/Documentation/arch/x86/xstate.rst
+++ b/Documentation/arch/x86/xstate.rst
@@ -172,3 +172,16 @@ are extended to control the guest permission:
 
 Note that some VMMs may have already established a set of supported state
 components. These options are not presumed to support any particular VMM.
+
+Signal Frame Portability
+------------------------
+
+The signal frame is designed to be self-describing and portable. This is
+especially important for checkpoint/restore tools like CRIU, which may restore
+a process on a different host than where it was checkpointed. A signal frame
+created on a machine with fewer CPU features can be successfully restored on a
+machine with more CPU features.
+
+Note that signal frame portability is generally guaranteed only between CPUs
+from the same vendor. Different vendors may use different offsets for the same
+xstate features in the xsave area, making frames incompatible between them.
diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index d0d9b331d3a1..d52313345f21 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -34,6 +34,19 @@
  * fpstate+extended_size-FP_XSTATE_MAGIC2_SIZE address) is set to
  * FP_XSTATE_MAGIC2 so that you can sanity check your size calculations.)
  *
+ * The xstate_size field indicates the actual size of the xstate context
+ * (including the fxregs_state and xstate_header). This size is used in
+ * conjunction with the pointer to the xstate context to locate
+ * FP_XSTATE_MAGIC2. Note that on 32-bit systems, the fpstate pointer points
+ * to a legacy struct fregs_state (112 bytes) that precedes the xstate
+ * context, so the xstate context starts at fpstate + 112. This makes
+ * the signal frame self-describing and portable: a signal frame created on a
+ * machine with a certain set of xstate features can be restored on a machine
+ * with a different (larger) set of features, as long as the latter supports
+ * all features present in the frame. Note that this portability is generally
+ * limited to CPUs of the same vendor, as different vendors may use different
+ * xstate layouts.
+ *
  * This extended area typically grows with newer CPUs that have larger and
  * larger XSAVE areas.
  */
-- 
2.54.0.1189.g8c84645362-goog