Date: Wed, 22 Mar 2017 08:36:08 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Thomas Garnier <thgarnie@google.com>
Cc: Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        kernel test robot <xiaolong.ye@intel.com>,
        Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@suse.de>, Chris Wilson <chris@chris-wilson.co.uk>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Jiri Kosina <jikos@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Jonathan Corbet <corbet@lwn.net>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>, Kees Cook <keescook@chromium.org>,
        Len Brown <len.brown@intel.com>, Lorenzo Stoakes <lstoakes@gmail.com>,
        "Luis R . Rodriguez" <mcgrof@kernel.org>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        Michal Hocko <mhocko@suse.com>, Paolo Bonzini <pbonzini@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Pavel Machek <pavel@ucw.cz>, Peter Zijlstra <peterz@infradead.org>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Rusty Russell <rusty@rustcorp.com.au>,
        Stanislaw Gruszka <sgruszka@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>, zijun_hu <zijun_hu@htc.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>, LKP <lkp@01.org>
Subject: Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot_stage
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* Thomas Garnier <thgarnie@google.com> wrote:

> >  static inline void setup_fixmap_gdt(int cpu)
> >  {
> >         __set_fixmap(get_cpu_gdt_ro_index(cpu),
> > -                    __pa(get_cpu_gdt_rw(cpu)), pg_fixmap_gdt_flags);
> > +                    slow_virt_to_phys(get_cpu_gdt_rw(cpu)),
> > +                    pg_fixmap_gdt_flags);
> >  }
> >
> >  /* Load the original GDT from the per-cpu structure */
> >
> > This makes UP boot for me, but SMP (2 cpus) is still busted.
> 
> This change fixed boot for me:
> 
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index b65155cc3760..4e30707d9f9a 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -104,7 +104,9 @@ enum fixed_addresses {
>         FIX_GDT_REMAP_BEGIN,
>         FIX_GDT_REMAP_END = FIX_GDT_REMAP_BEGIN + NR_CPUS - 1,
> 
> -       __end_of_permanent_fixed_addresses,
> +       __end_of_permanent_fixed_addresses =
> +               (FIX_GDT_REMAP_END + PTRS_PER_PTE - 1) &
> +               -PTRS_PER_PTE,
> 
> Just ensure PKMAP_BASE & FIX_WP_TEST are on a different PMD.
> 
> I don't think that the right fix but it might help understand the
> exact root cause.

Could this be related to the permission bits in the PMD itself getting out of sync 
with the PTEs? WP test marks a page writable/unwritable, and maybe we mess up the 
restoration. If they are on separate PMDs then this is worked around because the 
fixmap GDT is on a separate PMD.

Thanks,

	Ingo