linux-kernel - FPU warning on x86

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Date:   Thu, 24 Nov 2016 09:11:41 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: FPU warning on x86_32 on Skylake

I gett this when booting a 32-bit 4.9-rc6-ish on Skylake:

[    0.564506] ------------[ cut here ]------------
[    0.564994] WARNING: CPU: 0 PID: 1 at
./arch/x86/include/asm/fpu/internal.h:368 fpu__restore+0x203/0x210
[    0.565737] Modules linked in:
[    0.566040] CPU: 0 PID: 1 Comm: sh Not tainted 4.9.0-rc6+ #488
[    0.566502] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.9.3-1.fc25 04/01/2014
[    0.567174]  c78a9e5c c135a6d0 00000000 c1ac4b4c c78a9e8c c10aeb42
c1ad53f0 00000000
[    0.567896]  00000001 c1ac4b4c 00000170 c107e753 00000170 c78a06c0
00000000 c78a0700
[    0.568583]  c78a9ea0 c10aec05 00000009 00000000 00000000 c78a9eb8
c107e753 c78a0700
[    0.569245] Call Trace:
[    0.569440]  [<c135a6d0>] dump_stack+0x58/0x78
[    0.569783]  [<c10aeb42>] __warn+0xe2/0x100
[    0.570109]  [<c107e753>] ? fpu__restore+0x203/0x210
[    0.570519]  [<c10aec05>] warn_slowpath_null+0x25/0x30
[    0.570943]  [<c107e753>] fpu__restore+0x203/0x210
[    0.571312]  [<c107ff5c>] __fpu__restore_sig+0x1fc/0x580
[    0.571719]  [<c108050a>] fpu__restore_sig+0x2a/0x50
[    0.572103]  [<c107413d>] restore_sigcontext.isra.10+0xbd/0xd0
[    0.572546]  [<c1074a11>] sys_sigreturn+0x81/0x90
[    0.572908]  [<c1001837>] do_int80_syscall_32+0x57/0xc0
[    0.573306]  [<c190eb06>] entry_INT80_32+0x2a/0x2a
[    0.573677] ---[ end trace 88038c46b2a9d23a ]---

Telling KVM to disable XSAVES makes the warning go away.

I seem to be the only person testing 32-bit kernels on CPUs this new :-/

--Andy