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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 22, 2018 at 06:16:05PM +0200, Thomas Gleixner wrote:
> On Fri, 22 Jun 2018, Kirill A. Shutemov wrote:
> > On Fri, Jun 22, 2018 at 03:35:18PM +0000, Thomas Gleixner wrote:

> > > 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 static_cpu_has() _should_ work. That thing is mightily convoluted,
but behold:

| static __always_inline __pure bool _static_cpu_has(u16 bit)
| {
|         asm_volatile_goto("1: jmp 6f\n"
|                  "2:\n"
|                  ".skip -(((5f-4f) - (2b-1b)) > 0) * "
|                          "((5f-4f) - (2b-1b)),0x90\n"

<snip magic shite>

|                  ".section .altinstr_aux,\"ax\"\n"
|                  "6:\n"
|                  " testb %[bitnum],%[cap_byte]\n"
|                  " jnz %l[t_yes]\n"
|                  " jmp %l[t_no]\n"
|                  ".previous\n"
|                  : : [feature]  "i" (bit),
|                      [always]   "i" (X86_FEATURE_ALWAYS),
|                      [bitnum]   "i" (1 << (bit & 7)),
|                      [cap_byte] "m" (((const char *)boot_cpu_data.x86_capability)[bit >> 3])
|                  : : t_yes, t_no);
| t_yes:
|         return true;
| t_no:
|         return false;
| }

So by default that emits, before patching:

	jmp 6f
	'however many single byte NOPs are needed'

	.section.altinstr_aux
	6: testb %[bitnum],%[cap_byte]
	   jnz %l[t_yes]
	   jmp %l[t_no]
	.previous

Which is a dynamic test for the bit in the bitmask. Which always works,
irrespective of the alternative patching.

The magic, which I cut out, will rewrite the "jmp 6f, nops" thing to
"jmp %l[y_{yes,no}]" at the alternative patching and we'll loose the
dynamic test, pinning the condition forever more.