Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

[Tim Chen]

On 01/04/2018 02:54 PM, Peter Zijlstra wrote:
> On Thu, Jan 04, 2018 at 09:56:47AM -0800, Tim Chen wrote:
>>  .macro ENABLE_IBRS
>> -	ALTERNATIVE "jmp 10f", "", X86_FEATURE_SPEC_CTRL
>> +	testl $SPEC_CTRL_IBRS_INUSE, spec_ctrl_ibrs
>> +	jz .Lskip_\@
>> +
>>  	PUSH_MSR_REGS
>>  	WRMSR_ASM $MSR_IA32_SPEC_CTRL, $SPEC_CTRL_FEATURE_ENABLE_IBRS
>>  	POP_MSR_REGS
>> -10:
>> +
>> +	jmp .Ldone_\@
>> +.Lskip_\@:
>> +	/*
>> +	 * prevent speculation beyond here as we could want to
>> +	 * stop speculation by enabling IBRS
>> +	 */
>> +	lfence
>> +.Ldone_\@:
>>  .endm
> 
> 
> Yeah no. We have jump labels for this stuff. There is no reason what so
> ever to do dynamic tests for a variable that _never_ changes.
> 

Admin can change spec_ctrl_ibrs value at run time, or when
we scan new microcode.  So it doesn't often change, but it could.

There may be time when the admin wants to run the system
in a more secure mode, and time when it is okay to leave out
IBRS.

Tim