From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-oo1-f70.google.com (mail-oo1-f70.google.com [209.85.161.70]) (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 9A19D204C31 for ; Tue, 8 Sep 2026 04:34:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.161.70 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788842074; cv=none; b=IaCxcfQ2JOi42olVrqDUgXENsFKUP2gG8TCG1D8eGxp55NJOvlm0d8H+P/tQ6C/o+39jJxCOgcK9knUlrFgySZs8tkya1hZDnu5ghopFsBK3INi2w60TDktXusy8WbTEc3+SGD50d077xOyF6jP/tZQr4wOPDy4NWSC9aSgQk2I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788842074; c=relaxed/simple; bh=820WAcn/OhQr9WjFQhybPFw8BQBq2i5U1o+fO7Nb+VY=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=m+6n0F1B+ZymdhcIY607VjtQNm3mfdSA1mBjBUKhM4BTfR9lWHsCh9c1wCDWFMW3nwM4eAL7Nv72PeUDpbTI6WxiZ4sAb9fnxYbGP4oGCP+gwU7MVUtmng6r4fVSdpcpl51iEFwyC9Y+xkERf0BmsLYc2dSSNy+kELPap1Atius= 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=rj5GEUkd; arc=none smtp.client-ip=209.85.161.70 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="rj5GEUkd" Received: by mail-oo1-f70.google.com with SMTP id 006d021491bc7-6b352d9062fso4940634eaf.3 for ; Mon, 07 Sep 2026 21:34:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1788842071; x=1789446871; darn=vger.kernel.org; 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=K7eSdg298vcCEnyIcdSVrV4v+DIWuyNtu9DRp3wZxhc=; b=rj5GEUkdSRbKCn+fCB9/BIQ7JP90Dmn39WMQJd9vWpTAAaITv5ZPuLvwxFkKHnXGMJ cOsU7znUt1fEV2OAJ/oIA7SlWC8PbQQKchbnn8x3JEqLqOasp6GUAJPxBP1StTsq2jYT RfQOdpQswRaicuguoxbkPV2GYz/Ql07YgnTdWWBZOnDdH/8MtxD6Y8tyGIfJsDRdMAKC nLK9nvto25JYC4KtALY4fHDIOzneG4jQ8i8NHJ8Dapfs589gXFfz61jYyFpiHizzt5gT c9hADS+hfAW8OnTvlB7Zuq/b1ZUqH9wGraqPDgq2j6KH2Fs5v8/C/FDexth+PYUKeM79 MJZQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1788842071; x=1789446871; 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=K7eSdg298vcCEnyIcdSVrV4v+DIWuyNtu9DRp3wZxhc=; b=il+f8A0BcX5wUUs7vrnnOc49irvQA4a+IApfq6Vq5+pAFZ9rYxbnODzZOltlpo2J9d sPdpfcm7C8FWV25aRLyJYKIJQmmn8ovDMqjj1Dnvm+YJGllgCZ83JHgEyh5FaYYhX3M3 iHxgfH6LgD6xtxzyW40jaZu7ycTLXQFyGZ5n7Jp6YhATjd9K0a7qxO4AkFO19IWACuI8 nGoGBXkDm9HSMh0Z+hYs1ySn66fSWtomPU6A8U3Usa56cw/YSXgLrWvyz7wPbDeN9dL/ CcNm7cZJZF8rbRzVix901EROi/iJSaOU7FIL9V42SSHRMKVQmgIAOBxNn5DPsgSx8cr6 Xqpg== X-Gm-Message-State: AFuF++kAeh/EKEtcGtfIx64L5o0LRgnzSz8BYhIj2Ov8xe5h+/byw1EE ZwhS8YUxKnPm6o1J2nTkFJOwtDtn4F3MvvLu77KDAn+k3jSxSCKCqPW/cdgllmc4n4LesMWMjZ+ G5oSi7g== X-Received: from ilbbn26.prod.google.com ([2002:a05:6e02:339a:b0:50b:4ac:93e6]) (user=avagin job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6820:2d08:b0:6b1:a812:9871 with SMTP id 006d021491bc7-6b6fbbdb3cbmr16624710eaf.18.1788842071107; Mon, 07 Sep 2026 21:34:31 -0700 (PDT) Date: Tue, 8 Sep 2026 04:34:21 +0000 In-Reply-To: <20260908043427.1842515-1-avagin@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260908043427.1842515-1-avagin@google.com> X-Mailer: git-send-email 2.55.0.979.g7e5102b832-goog Message-ID: <20260908043427.1842515-2-avagin@google.com> Subject: [PATCH 1/7] x86/fpu: Document signal frame layout and portability From: Andrei Vagin To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "Chang S. 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. Document the signal frame software reserved bytes (struct _fpx_sw_bytes) and portability constraints in Documentation/arch/x86/xstate.rst, and add a summary and cross-reference in . Reviewed-by: Alexander Mikhalitsyn Signed-off-by: Andrei Vagin --- Documentation/arch/x86/xstate.rst | 55 ++++++++++++++++++++++++++ arch/x86/include/uapi/asm/sigcontext.h | 14 +++++++ 2 files changed, 69 insertions(+) diff --git a/Documentation/arch/x86/xstate.rst b/Documentation/arch/x86/xstate.rst index cec05ac464c1..92e5ff7dd6a2 100644 --- a/Documentation/arch/x86/xstate.rst +++ b/Documentation/arch/x86/xstate.rst @@ -172,3 +172,58 @@ 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. + +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 (including the trailing + ``FP_XSTATE_MAGIC2`` marker and any alignment padding). In 32-bit signal + frames, this also includes the 112-byte legacy ``struct _fpstate_32`` + prefix. +- ``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 a legacy 112-byte FPU environment (``struct _fpstate_32``) +that precedes the xstate context, so 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..d85226c67e19 100644 --- a/arch/x86/include/uapi/asm/sigcontext.h +++ b/arch/x86/include/uapi/asm/sigcontext.h @@ -34,6 +34,20 @@ * 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 a legacy 112-byte FPU environment + * (struct _fpstate_32) that precedes the xstate context, 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. */ -- 2.55.0.979.g7e5102b832-goog