linux-kernel - Re: [PATCH REBASED RESEND] x86/cpu: Move early cpu initialization into a separate translation unit

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Date:   Fri, 22 Jun 2018 18:16:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
cc:     Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH REBASED RESEND] x86/cpu: Move early cpu initialization
 into a separate translation unit

On Fri, 22 Jun 2018, Kirill A. Shutemov wrote:
> On Fri, Jun 22, 2018 at 03:35:18PM +0000, Thomas Gleixner wrote:
> > > > Second thoughts.
> > > > 
> > > > The only place where __pgtable_l5_enabled() is used in common.c is in
> > > > early_identify_cpu() which is marked __init. So how is that section
> > > > mismatch triggered?
> > > 
> > > Yeah, it's not obvious:
> > > 
> > > cpu_init()
> > >   load_mm_ldt()
> > >     ldt_slot_va()
> > >       LDT_BASE_ADDR
> > >         LDT_PGD_ENTRY
> > > 	  pgtable_l5_enabled()
> > 
> > How is that supposed to work correctly?
> > 
> > start_kernel()
> >   ....
> >   trap_init()
> >     cpu_init()
> > 
> >   ....
> >   check_bugs()
> >     alternative_instructions()
> > 
> > So the first invocation of cpu_init() on the boot CPU will then use
> > static_cpu_has() which is not yet initialized proper.
> 
> Ouch.
> 
> Is there a way to catch such improper static_cpu_has() users?
> Silent misbehaviour is risky.

Yes, it is. I don't think we have something in place right now, but we
should add it definitely. PeterZ ????

> > So, no. That does not work and the proper fix is:
> > 
> > -unsigned int __pgtable_l5_enabled __initdata;
> > +unsigned int __pgtable_l5_enabled __ro_after_init;
> > 
> > and make cpu/common.c use the early variant. The extra 4 bytes storage are
> > not a problem and cpu_init() is not a fast path at all.
> 
> Okay, I'll prepare the patch.
> 
> BTW, if we go this path after all, shouldn't we revert these:
> 
>   046c0dbec023 ("x86: Mark native_set_p4d() as __always_inline")
>   1ea66554d3b0 ("x86/mm: Mark p4d_offset() __always_inline")

In principle the always inline is fine, but the changelogs are quite
misleading and I really regret that I did not take the time to analyse that
proper when I applied the patches. At least we have catched it now.

So yes, please send the reverts along. Can you please add a proper root
cause analysis for the issues Arnd has observed to the changelogs so we
have it documented for later.

Thanks,

	tglx