Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

[Konrad Rzeszutek Wilk <konrad@darnok.org>]

On Mon, Jan 08, 2018 at 05:14:37PM +0100, Peter Zijlstra wrote:
> On Mon, Jan 08, 2018 at 01:47:07PM +0100, Peter Zijlstra wrote:
> > > a good suggestion, but we encountered some issues with it either
> > > crashing the kernel at boot or not properly turning on/off.
> 
> The below boots, but I lack stuff to test the enabling.

..snip..
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -373,22 +373,17 @@ For 32-bit we have the following convent
>  .endm
>  
>  .macro ENABLE_IBRS
> -	testl	$1, dynamic_ibrs
> -	jz	.Lskip_\@
> +	STATIC_JUMP_IF_FALSE .Lskip_\@, ibrs_key, def=0
>  
>  	PUSH_MSR_REGS
>  	WRMSR_ASM $MSR_IA32_SPEC_CTRL, $SPEC_CTRL_FEATURE_ENABLE_IBRS
>  	POP_MSR_REGS
> -	jmp	.Ldone_\@
>  
>  .Lskip_\@:
> -	lfence
> -.Ldone_\@:
>  .endm

I know that this particular patchset is now obsolete as the retpoline
along with stuffing the RSB half or full is the preferred way.

But I am wondering - why was the 'lfence' added in the first place
if dynamic_ibrs was zero?

It certainly is not putting the speculative execution on a wild ride
like: "[tip:x86/pti] x86/retpoline: Use LFENCE instead of PAUSE in the
retpoline/RSB filling RSB macros" https://git.kernel.org/tip/2eb9137c8744f9adf1670e9aa52850948a30112b

So what was the intent behind this? Was it: "oh if we do not have
IBRS let us at least add lfence on  every system call, interrupt, nmi,
exception, etc to do a poor man version of IBRS?"

Thank you.
P.S.
My apologies if this was discussed in the prior versions of this thread.
I must have missed it.