From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-oi1-f199.google.com (mail-oi1-f199.google.com [209.85.167.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CFD9F4DAF9F for ; Fri, 25 Sep 2026 16:24:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.167.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790353500; cv=none; b=H3yLKzkq6f+JGgnLPL6P8M7noGj/xa15d7YTWYNoO7YLGYxXmyZsHXvovhLuZgVm5o35sTwPPprEJads5nueF1MeAOqDuHcMMF+2eAma7Ix0GCfCuqhk7K+l5wye6mIRMxowD8U9jNl4IbAzo9KeBelIA45QKE/8H4t4Hz4TP68= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790353500; c=relaxed/simple; bh=MFHgjRUFrURWMuBfH/7m+QQEwb6KSPJcfIBqBGQPZlI=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=a8HPjaANXXCCfhaIY8rsuDiJQDNB1DTTANoj1++fuYFWllzY6xtn0JHKkLWuAs0CCU7JsQjTOna2vr7RJn+0mzSzEChY6vBEGrwSq1RLabGTvhgV6zFSHZey7bG8b+glXDfyyLybAAYYcDHKEkJ9EyM8malCYKI9cTfb122SAXw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--avagin.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=wRvdZkoU; arc=none smtp.client-ip=209.85.167.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--avagin.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="wRvdZkoU" Received: by mail-oi1-f199.google.com with SMTP id 5614622812f47-4b38ea4c6acso1097091b6e.1 for ; Fri, 25 Sep 2026 09:24:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1790353498; x=1790958298; darn=lists.linux.dev; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=VWY43kpRw2o5ypwDPUlrvSnRFUL6IMLWNwvSOplpeh0=; b=wRvdZkoUrXrJVWPGCjdIRiX3vTCi0RxRC8nhIhw/Szfyn8uEkYc02+JM2F8divZh4K /8SOizUBJHUiLEf4+56i+7XgyrxsWIOMw/Vvfsl5ILuTVxnRE2GwmgF0qqZGdo6yC4Wm m55nA91AzvKmbLUipmcIZm3QFfpSOXjE8/pB0YQo4UZ/Qxy+PDhi0I7dGY4+ry4bqbOR gLDdfMlRK7UtU+ppCcbXu9q/Cqdz7DGhjTMqHCBFOzRw5Lt/OeliXL8qIabHlVqV/Sn8 6/XKUa2HY3x/qTt2JsZ/21K85KltYR36CEqIoP55tFGBcZakASOz3mEu/sCUnsh4vb6c qD8Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1790353498; x=1790958298; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=VWY43kpRw2o5ypwDPUlrvSnRFUL6IMLWNwvSOplpeh0=; b=cBlqvfm32AXg64C6qRzVVHlkNrKkrkTOz8EYwMe0mSbPDVn7cxu3viGqRo0Fk4Ht3y Ta0xs7zZDzcZYZ2kZC+gmZ5VupIfgSG87gmLYdm6BBAx+KRKeIBewLMRn+WbgJMe/sqj hmWHdjS58jRX91hu15KDDUUfBAdE0pGZPwxKemCpRCXALk9SdcukNnHf4b/FNcpNdTCj tohZX0O8JGOD+Q4y3bfIVJIDRM9QiJOkUAPcOV1QCaayDiFBSj87sTmM85dt44b7bXyr hn8V2NFZNBOWKyPTy9qnU4Y5nMIEit07uc98H7IoDepXnUbRJqZaiHU44K94kFYCjKYS wNtg== X-Forwarded-Encrypted: i=1; AKwUvBxwcpO9ZAZAS6LmsKK8EGPyAxCoVJgN0nB1Etp/iQ0rOT2K2jDAEOW4q75HsTvIovRdV3Gp@lists.linux.dev X-Gm-Message-State: AFuF++lMesagauZ2wttkjkExPEK2RXRpq62de27rTb50unO4Dg/IF8J3 JoWvYwdgYLf0RQtnbYQ4729r0gyPZ2eLqBgiJO95VM5hfW0o841LyyEJLAuun6QSUd2kBCu+54x Pi72BRw== X-Received: from iobdk26.prod.google.com ([2002:a05:6602:1b1a:b0:9c4:2378:b447]) (user=avagin job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6808:c3f4:b0:4d6:9273:251d with SMTP id 5614622812f47-4d72cf4d5abmr7256779b6e.57.1790353497434; Fri, 25 Sep 2026 09:24:57 -0700 (PDT) Date: Fri, 25 Sep 2026 16:24:47 +0000 In-Reply-To: <20260925162454.1403405-1-avagin@google.com> Precedence: bulk X-Mailing-List: criu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260925162454.1403405-1-avagin@google.com> X-Mailer: git-send-email 2.56.0.rc1.315.gc6ed9934b7-goog Message-ID: <20260925162454.1403405-2-avagin@google.com> Subject: [PATCH 1/7] x86/fpu: Document signal frame layout and portability From: Andrei Vagin To: Borislav Petkov , "Chang S. Bae" Cc: linux-kernel@vger.kernel.org, criu@lists.linux.dev, Thomas Gleixner , Ingo Molnar , 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