Date: Thu, 14 Mar 2024 16:53:27 +0530
From: Vignesh Balasubramanian <vigbalas@....com>
To: <linux-kernel@...r.kernel.org>, <linux-toolchains@...r.kernel.org>
CC: <mpe@...erman.id.au>, <npiggin@...il.com>, <christophe.leroy@...roup.eu>,
	<aneesh.kumar@...nel.org>, <naveen.n.rao@...ux.ibm.com>,
	<ebiederm@...ssion.com>, <keescook@...omium.org>, <x86@...nel.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-mm@...ck.org>, <bpetkov@....com>,
	<jinisusan.george@....com>, <matz@...e.de>, <binutils@...rceware.org>,
	<jhb@...eBSD.org>, <felix.willgerodt@...el.com>, Vignesh Balasubramanian
	<vigbalas@....com>
Subject: [PATCH 0/1] Add XSAVE layout description to Core files for debuggers to support varying XSAVE layouts

This patch proposes to add an extra .note section in the corefile to dump the CPUID information of a machine. This is being done to solve the issue of tools like the debuggers having to deal with coredumps from machines with varying XSAVE layouts in spite of having the same XCR0 bits. The new proposed .note section, at this point, consists of an array of records containing the information of each extended feature that is present. This provides details about the offsets and the sizes of the various extended save state components of the machine where the application crash occurred. Requesting a review for this patch.

Some background:

The XSAVE layouts of modern AMD and Intel CPUs differ, especially since Memory Protection Keys and the AVX-512 features have been inculcated into the AMD CPUs. This is since AMD never adopted (and hence never left room in the XSAVE layout for) the Intel MPX feature. Tools like GDB had assumed a fixed XSAVE layout matching that of Intel (based on the XCR0 mask). Hence, the core dumps from AMD CPUs didn't match the known size for the XCR0 mask. This resulted in GDB and other tools not being able to access the values of the AVX-512 and PKRU registers on AMD CPUs. To solve this, an interim solution has been accepted into GDB, and is already a part of GDB 14, thanks to these series of patches : [ https://sourceware.org/pipermail/gdb-patches/2023-March/198081.html ].
But this patch series depends on heuristics based on the total XSAVE register set size and the XCR0 mask to infer the layouts of the various register blocks for core dumps, and hence, is not a foolproof mechanism to determine the layout of the XSAVE area.

Hence this new core dump note has been proposed as a more sturdy mechanism to allow GDB/LLDB and other relevant tools to determine the layout of the XSAVE area of the machine where the corefile was dumped.
The  new core dump note (which is being proposed as a per-process .note section), NT_X86_XSAVE_LAYOUT (0x205) contains an array of structures.
Each structure describes an individual extended feature containing offset, size and flags (that is obtained through CPUID instruction) in a format roughly matching the follow C structure:

struct xfeat_component {
       u32 xfeat_type;
       u32 xfeat_sz;
       u32 xfeat_off;
       u32 xfeat_flags;
};


Vignesh Balasubramanian (1):
  x86/elf: Add a new .note section containing Xfeatures information to
    x86 core files

 arch/Kconfig                   |   9 +++
 arch/powerpc/Kconfig           |   1 +
 arch/powerpc/include/asm/elf.h |   2 -
 arch/x86/Kconfig               |   1 +
 arch/x86/include/asm/elf.h     |   7 +++
 arch/x86/kernel/fpu/xstate.c   | 101 +++++++++++++++++++++++++++++++++
 include/linux/elf.h            |   2 +-
 include/uapi/linux/elf.h       |   1 +
 8 files changed, 121 insertions(+), 3 deletions(-)

-- 
2.43.0

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