linux-kernel - Re: AMD DC graphics display code enables -mhard-float, -msse, -msse2 without any visible FPU state protection

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Message-ID: <20200417215601.GQ2483@worktop.programming.kicks-ass.net>
Date:   Fri, 17 Apr 2020 23:56:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rodrigo Siqueira <Rodrigo.Siqueira@....com>
Cc:     Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jann Horn <jannh@...gle.com>, Leo Li <sunpeng.li@....com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Alex Deucher <alexander.deucher@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Harry Wentland <harry.wentland@....com>, mhiramat@...nel.org
Subject: Re: AMD DC graphics display code enables -mhard-float, -msse, -msse2
 without any visible FPU state protection

On Fri, Apr 17, 2020 at 04:27:28PM -0400, Rodrigo Siqueira wrote:
> I'm going to work on the FPU issues in the display code. In this sense,
> could you clarify a little bit more about the Makefile issues?

I think it might have been PEBKAC, I assumed allmodconfig would end up
selecting the driver, it doesn't!

Adding "AMD GPU" to a defconfig seems to make it work:

$ make O=defconfig-build/ drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o

Sorry about the confusion.

> Also, I applied the patch `[PATCH v4] x86: insn: Add insn_is_fpu()` and
> tried to reproduce the warning that you described but I didn't see it.
> Could you explain to me how I can check those warnings?

grab:

  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/fpu

Then build like above:

$ make O=defconfig-build/ drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o

And manually validate:

$ defconfig-build/tools/objtool/objtool check -Ffa defconfig-build/drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o
defconfig-build/drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o: warning: objtool: dcn_validate_bandwidth()+0x1b17: FPU instruction outside of kernel_fpu_{begin,end}()