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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 29 Dec 2017 12:34:42 -0800
Message-ID: <CA+55aFxEQm2DCUFZ4aS=EEsk9uBSDwZo0ExFsQjPRcHcX-R5kw@mail.gmail.com>
Subject: Re: 4.14.9 with CONFIG_MCORE2 fails to boot
To: Alexander Tsoy <alexander@tsoy.me>
Cc: Dave Hansen <dave.hansen@intel.com>, Greg KH <greg@kroah.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@alien8.de>, Borislav Petkov <bpetkov@suse.de>,
        Brian Gerst <brgerst@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Laight <David.Laight@aculab.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Greg KH <gregkh@linuxfoundation.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>, Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>, Rik van Riel <riel@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        "Liguori, Anthony" <aliguori@amazon.com>,
        Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
        Hugh Dickins <hughd@google.com>, Kees Cook <keescook@google.com>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        stable <stable@vger.kernel.org>
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On Fri, Dec 29, 2017 at 12:22 PM, Alexander Tsoy <alexander@tsoy.me> wrote:
>> But double-checking that "-march=core2" case is definitely worth
>> looking into. Especially since there are clear indications that it's
>> gcc version-dependent anyway. Alexander?
>
> Yes, the change suggested by Dave makes the problem go away.

Ok, that's good information.

It doesn't really explain *why* that commit 7f2590a110b8
("x86/entry/64: Use a per-CPU trampoline stack for IDT entries") ends
up being sensitive to that compiler option, though.

So it narrows the cause down, but it doesn't really root-cause the
problem. It tends to be almost impossible to find differences in code
generation, because they are generally all over.

Ho humm. What happens if you change the "-march=core2" to
"-mtune=core2"? Does it still boot?

Because maybe the actual differences that "-march=core2" generates
might be easier to see when compared to "-mtune=core2".

                  Linus