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

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