linux-kernel - Re: [RFD] x86: The future of MPX

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Date:   Sat, 28 Apr 2018 11:18:23 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFD] x86: The future of MPX

* Thomas Gleixner <tglx@...utronix.de> wrote:

> Dave,
> 
> I've got the following MPX related information:
> 
>  .... But they [Intel] certainly haven't maintained the whole MPX support
>  in GCC at all since two or three years (e.g. bootstrap with MPX was broken
>  since forever), so now it's gone from GCC. ....
> 
> and I know that GCC is discussing MPX removal since May last year at least.
> To the best of my knowledge clang does not support it either.
> 
> So what's the state of this stuff? If we don't have a compiler supporting
> it how on earth is that useful or even testable for anyone who does not
> have access to some magic Intel internal compiler version?
> 
> If that's the sad state of affairs, then we just should move the whole
> stuff into staging or simply queue it for removal in 4.18.

I just tried the MPX testcases with the latest kernel:

 deimos:~/tip/tools/testing/selftests/x86> ./mpx-mini-test_64 
 XSAVE is supported by HW & OS
 XSAVE processor supported state mask: 0x21f
 XSAVE OS supported state mask: 0x21f
  BNDREGS: size: 64 user: 1 supervisor: 0 aligned: 0
   BNDCSR: size: 64 user: 1 supervisor: 0 aligned: 0
 mpx-mini-test_64: mpx-mini-test.c:58: write_int_to: Assertion `fd >= 0' failed.
 Aborted (core dumped)

 deimos:~/tip/tools/testing/selftests/x86> ./mpx-mini-test_32 
 XSAVE is supported by HW & OS
 XSAVE processor supported state mask: 0x21f
 XSAVE OS supported state mask: 0x21f
  BNDREGS: size: 64 user: 1 supervisor: 0 aligned: 0
   BNDCSR: size: 64 user: 1 supervisor: 0 aligned: 0
 mpx-mini-test_32: mpx-mini-test.c:58: write_int_to: Assertion `fd >= 0' failed.
 Aborted (core dumped)

... and both of them crashed.

The CPU has MPX support and the kernel has CONFIG_X86_INTEL_MPX=y:

 deimos:~> dmesg | grep -i mpx
 [    0.000000] x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registers'
 [    0.000000] x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'

Thanks,

	Ingo