On Fri, Jun 22, 2018 at 03:35:18PM +0000, Thomas Gleixner wrote:
> On Fri, 22 Jun 2018, Kirill A. Shutemov wrote:
> > On Fri, Jun 22, 2018 at 02:05:47PM +0000, Thomas Gleixner wrote:
> > > On Tue, 12 Jun 2018, Kirill A. Shutemov wrote:
> > >
> > > > __pgtable_l5_enabled shouldn't be needed after system has booted, we can
> > > > mark it as __initdata, but it requires preparation.
> > > >
> > > > This patch moves early cpu initialization into a separate translation
> > > > unit. This limits effect of USE_EARLY_PGTABLE_L5 to less code.
> > > >
> > > > Without the change cpu_init() uses __pgtable_l5_enabled. cpu_init() is
> > > > not __init function and it leads to section mismatch.
> > > >
> > > > Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
> > > > Reviewed-by: Thomas Gleixner <t...@linutronix.de>
> > >
> > > 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.
> 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")
?
I can send it as part of the patchset.
--
Kirill A. Shutemov
