linux-kernel - Re: [PATCH] x86: work around clang IAS bug referencing __force

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Message-ID: <CAKwvOdnj6ObdpsdVYkDxWp-dVTTg=xMkBm84y419SNtLuAqfMg@mail.gmail.com>
Date:   Fri, 14 Aug 2020 15:57:31 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Fri, Aug 14, 2020 at 2:19 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> On Fri, Aug 14, 2020 at 7:29 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > Thanks for the proposal.
> >
> > I have adapted it to fit my patchset against Linux v5.8.
> >
> > Both Debian's GCC-10 and a snapshot version of LLVM toolchain
> > v11.0.0-rc1+ seems to be OK.
> >
>
> Yupp, OK.
>
> I was able to boot FreeDOS 1.2 VM in VirtualBox GUI.

Hi Sedat,
Apologies, but it's not clear to me precisely which patch you tested.
Can you please confirm whether you tested:
1. Arnd's patch that started this thread.
2. My proposed diff adding -fno-addrsig to CFLAGS_powernow-k6.o.
3. My proposed diff removing __force_order from the kernel.

I'm hoping you were referring to testing 3., but it's not clear to me.
I've been comparing the full disassemblies of vmlinux images when
built with Clang with 3 applied (they're no different, which is a
pleasant surprise, I didn't think kernel builds woulds would be fully
deterministic given the sheer amount of source).  I still need to
check the compressed vmlinux image, and various .ko's (XEN) that use
these read/write_cr[0,1,2,4]() functions, and then check them again
when built with GCC.  I'm falling behind a little trying to get our MC
organized for plumbers, as well as the end of intern season and
beginning of bi-annual "performance review" ("not stack ranking" I'm
told) at work.  If I don't find any differences, or if I do but don't
find them to be meaningful, I hope to push a more formal patch (rather
than just a diff) maybe next week.  I'll include my findings either
way; if it was 3 that you tested, I'll include your tested by tag when
sending.  Otherwise maybe you can help us test the more formal patch
next week?
-- 
Thanks,
~Nick Desaulniers