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Bae" Cc: linux-kernel@vger.kernel.org, criu@lists.linux.dev, Dave Hansen , x86@kernel.org, Andrei Vagin , Alexander Mikhalitsyn , "H. Peter Anvin" Content-Type: text/plain; charset="UTF-8" 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. Reviewed-by: Alexander Mikhalitsyn Signed-off-by: Andrei Vagin --- Documentation/arch/x86/xstate.rst | 54 ++++++++++++++++++++++++++ arch/x86/include/uapi/asm/sigcontext.h | 15 +++++++ 2 files changed, 69 insertions(+) diff --git a/Documentation/arch/x86/xstate.rst b/Documentation/arch/x86/xstate.rst index cec05ac464c1..e2944f744255 100644 --- a/Documentation/arch/x86/xstate.rst +++ b/Documentation/arch/x86/xstate.rst @@ -172,3 +172,57 @@ 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 Layout and 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, but not vice-versa. + +Signal Frame Software Reserved Bytes +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +On CPUs supporting XSAVE, bytes 464..511 in the 512-byte FXSAVE/FXRSTOR frame +are reserved for software use and contain ``struct _fpx_sw_bytes`` (defined in +````):: + + struct _fpx_sw_bytes { + __u32 magic1; + __u32 extended_size; + __u64 xfeatures; + __u32 xstate_size; + __u32 padding[7]; + }; + +- ``magic1``: Set to ``FP_XSTATE_MAGIC1`` (``0x46505853U``) if an extended + xstate context is present; 0 for a legacy frame. +- ``extended_size``: The total size allocated on the stack for the frame, + measured from the ``fpstate`` pointer. In 32-bit signal frames, this also + includes the 112-byte legacy FPU state prefix of ``struct _fpstate_32``. +- ``xfeatures``: The mask of xstate features saved in the frame. +- ``xstate_size``: The actual size of the xstate context for the enabled + features (including the 512-byte FXSAVE area and the 64-byte XSAVE header). + +The kernel uses ``xstate_size`` in conjunction with the pointer to the xstate +context to locate the ``FP_XSTATE_MAGIC2`` (``0x46505845U``) marker right after +the xstate context (at ``xstate_context + xstate_size``). In 64-bit signal frames, +the ``fpstate`` pointer points directly to the xstate context. In 32-bit signal +frames (including 32-bit compat tasks on 64-bit kernels), the ``fpstate`` +pointer points to ``struct _fpstate_32``, which contains the 112-byte legacy +FPU state followed by the 512-byte FXSR state (and any extended xstate). Since +there is no standalone UAPI structure defined for just the 112-byte legacy +state, the xstate context starts at ``fpstate + 112`` (and ``extended_size`` +spans the entire allocation from ``fpstate``). + +Portability Constraints +^^^^^^^^^^^^^^^^^^^^^^^ + +Signal frame portability is constrained by the architectural XSAVE layout. +Restoration is supported only if the destination host supports all features +present in the frame and uses matching component offsets and sizes for them. +While layout compatibility is generally maintained across CPUs from the same +vendor, differences can occur across vendors or if the XSAVE space of a +deprecated feature (e.g. MPX) is repurposed for a newer feature (e.g. APX). diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h index d0d9b331d3a1..cff01406c0f4 100644 --- a/arch/x86/include/uapi/asm/sigcontext.h +++ b/arch/x86/include/uapi/asm/sigcontext.h @@ -34,6 +34,21 @@ * 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 512-byte FXSAVE area and the 64-byte XSAVE header struct + * _header). This size is used in conjunction with the pointer to the xstate + * context to locate FP_XSTATE_MAGIC2. + * + * In 64-bit signal frames, the fpstate pointer points directly to the xstate + * context. In 32-bit signal frames (including 32-bit compat tasks on 64-bit + * kernels), the fpstate pointer points to struct _fpstate_32, which contains + * the 112-byte legacy FPU state followed by the 512-byte FXSR state (and any + * extended xstate), so the xstate context starts at fpstate + 112. + * + * This makes the signal frame self-describing and portable across machines + * with different xstate features. See Documentation/arch/x86/xstate.rst + * for details on signal frame portability and its architectural constraints. + * * This extended area typically grows with newer CPUs that have larger and * larger XSAVE areas. */ -- 2.56.0.rc1.315.gc6ed9934b7-goog