Messages in this thread | | | Date | Fri, 22 Jun 2018 18:36:35 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH REBASED RESEND] x86/cpu: Move early cpu initialization into a separate translation unit |
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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.
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