Re: Kernel broken on processors without performance counters

[Jason Baron <jasonbaron0@gmail.com>]

On 07/21/2015 02:15 PM, Borislav Petkov wrote:
> On Tue, Jul 21, 2015 at 06:12:15PM +0200, Peter Zijlstra wrote:
>> Yes, if you start out false, you must be unlikely. If you start out
>> true, you must be likely.
>>
>> We could maybe try and untangle that if there really is a good use case,
>> but this is the current state.
>>
>> The whole reason this happened is because 'false' is like:
>>
>>
>> 	...
>> 	<nop>
>> 1:
>> 	...
>>
>>
>>
>> label:
>> 	<unlikely code>
>> 	jmp 1b
>>
>>
>> Where the code if out-of-line by default. The enable will rewrite the
>> <nop> with a jmp label.
> Btw, native_sched_clock() is kinda botched because of that, see below.
>
> I'd want that RDTSC to come first with a NOP preceding it which can
> become a JMP in case some idiotic CPU can't do RDTSC and needs to use
> jiffies. Instead, we *unconditionally* jump to RDTSC which is WTF?! We
> can just as well do a normal unlikely() without the static_key:

hmmm...so this is a case where need to the default the branch
to the out-of-line branch at boot. That is, we can't just enable
the out-of-line branch at boot time, b/c it might be too late at
that point? IE native_sched_clock() gets called very early?

Thanks,

-Jason

>
> 	.globl	native_sched_clock
> 	.type	native_sched_clock, @function
> native_sched_clock:
> 	pushq	%rbp	#
> 	movq	%rsp, %rbp	#,
> #APP
> # 21 "./arch/x86/include/asm/jump_label.h" 1
> 	1:.byte 0x0f,0x1f,0x44,0x00,0
> 	.pushsection __jump_table,  "aw" 
> 	 .balign 8 
> 	 .quad 1b, .L122, __use_tsc 	#,
> 	.popsection 
> 	
> # 0 "" 2
> #NO_APP
> 	movabsq	$-4294667296000000, %rax	#, tmp118
> 	popq	%rbp	#
> 	imulq	$1000000, jiffies_64(%rip), %rdx	#, jiffies_64, D.28443
> 	addq	%rdx, %rax	# D.28443, D.28443
> 	ret
> .L122:
> #APP
> # 118 "./arch/x86/include/asm/msr.h" 1
> 	rdtsc
> # 0 "" 2
>