Re: [PATCH v2 2/8] x86/enter: MACROS to set/clear IBRS

[Tim Chen <tim.c.chen@linux.intel.com>]

On 01/07/2018 04:03 AM, Borislav Petkov wrote:
> On Fri, Jan 05, 2018 at 06:12:17PM -0800, Tim Chen wrote:
> 
>> Subject: Re: [PATCH v2 2/8] x86/enter: MACROS to set/clear IBRS
> 
> Your subject needs to have a verb and not scream:
> 
> Subject: [PATCH v2 2/8] x86/entry: Add macros to set/clear IBRS
> 
>> Create macros to control IBRS.  Use these macros to enable IBRS on kernel entry
>> paths and disable IBRS on kernel exit paths.
>>
>> The registers rax, rcx and rdx are touched when controlling IBRS
>> so they need to be saved when they can't be clobbered.
>>
>> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
>> ---
>>  arch/x86/entry/calling.h | 74 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 74 insertions(+)
>>
>> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
>> index 45a63e0..09c870d 100644
>> --- a/arch/x86/entry/calling.h
>> +++ b/arch/x86/entry/calling.h
>> @@ -6,6 +6,8 @@
>>  #include <asm/percpu.h>
>>  #include <asm/asm-offsets.h>
>>  #include <asm/processor-flags.h>
>> +#include <asm/msr-index.h>
>> +#include <asm/cpufeatures.h>
>>  
>>  /*
>>  
>> @@ -347,3 +349,75 @@ For 32-bit we have the following conventions - kernel is built with
>>  .Lafter_call_\@:
>>  #endif
>>  .endm
>> +
>> +/*
>> + * IBRS related macros
>> + */
>> +.macro PUSH_MSR_REGS
>> +	pushq	%rax
>> +	pushq	%rcx
>> +	pushq	%rdx
>> +.endm
>> +
>> +.macro POP_MSR_REGS
>> +	popq	%rdx
>> +	popq	%rcx
>> +	popq	%rax
>> +.endm
>> +
>> +.macro WRMSR_ASM msr_nr:req eax_val:req
> 
> WRMSR as a name is good enough.
> 
> Also, you need edx_val:req too in case we decide to reuse that macro
> for something else later. Which I'm pretty sure we will, once it is out
> there.
> 
>> +	movl	\msr_nr, %ecx
>> +	movl	$0, %edx
> 
> ... and then
> 
> 	movl	\edx_val, %edx
> 
>> +	movl	\eax_val, %eax
>> +.endm
>> +
>> +.macro ENABLE_IBRS
>> +	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_SPEC_CTRL
>> +	PUSH_MSR_REGS
>> +	WRMSR_ASM $MSR_IA32_SPEC_CTRL, $SPEC_CTRL_FEATURE_ENABLE_IBRS
> 
> This is overwriting the previous contents of the MSR. You need to read
> it and OR-in its bits [63:2] with SPEC_CTRL_FEATURE_ENABLE_IBRS and
> clear bit 0.
> 
> Unless the rest of this MSR is not going to be used for anything else.
> Then you're fine.
> 

Currently we are not using other bits.
When the time comes that we have other bits in this MSR used, we will
change this.

Tim