linux-kernel - Re: FPU warning on x86

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Date:   Mon, 19 Dec 2016 16:53:18 -0500
From:   lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: FPU warning on x86_32 on Skylake

On Thu, Nov 24, 2016 at 09:11:41AM -0800, Andy Lutomirski wrote:
> 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 :-/

Well skylake added XRSTORS, which is used in place of XRSTOR if supported
by the CPU, but XRSTORS requires CPL=0, which XRSTOR did not as far as
I can tell.  Older CPUs don't have XRSTORS so this would not be an
issue there.

I don't know, but would not be surprised if running under kvm means the
guest kernel is not running with CPL=0 and hence the XRSTORS feature
ought not to be exposed as supported by the CPU to the guest kernel.

Just a guess.

Does this happen with a 64 bit kvm guest too?  Does it happen if the 32
bit kernel is booted on bare hardware?  My guess if I am thinking the
right thing is that the answers are yes and no respectively.

Looks like this was hit in jvm a couple of years ago:
https://www.spinics.net/lists/kvm/msg110434.html 

No idea what the resolution was if any.

-- 
Len Sorensen