Re: [PATCH 1/2] x86/bugs: Check VERW mitigations for consistency

[Daniel Sneddon]

On 10/29/24 09:32, Nikolay Borisov wrote:
> 
> 
> On 29.10.24 г. 1:50 ч., Daniel Sneddon wrote:
>> There are currently 4 mitigations that use VERW: MDS, TAA,
>> MMIO Stale Data, and Register File Data Sampling. Because
>> all 4 use the same mitigation path, if any one of them is
>> enabled, they're all enabled. Normally, this is what is
>> wanted. However, if a user wants to disable the mitigation,
>> this can cause problems. If the user misses disabling even
>> one of these mitigations, then none of them will be
>> disabled. This can cause confusion as the user expects to
>> regain the performance lost to the mitigation but isn't
>> seeing any improvement. Since there are already 4 knobs for
>> controlling it, adding a 5th knob that controls all 4
>> mitigations together would just overcomplicate things.
>> Instead, let the user know their mitigations are out of sync
>> when at least one of these mitigations is disabled but not
>> all 4.
>>
>> Signed-off-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
>> ---
>>   arch/x86/kernel/cpu/bugs.c | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>>
>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>> index d1915427b4ff..b26b3b554330 100644
>> --- a/arch/x86/kernel/cpu/bugs.c
>> +++ b/arch/x86/kernel/cpu/bugs.c
>> @@ -582,8 +582,26 @@ static void __init md_clear_update_mitigation(void)
>>   		pr_info("Register File Data Sampling: %s\n", rfds_strings[rfds_mitigation]);
>>   }
>>   
>> +static void __init verw_mitigations_check(void)
>> +{
>> +	if (mds_mitigation == MDS_MITIGATION_OFF ||
>> +	    taa_mitigation == TAA_MITIGATION_OFF ||
>> +	    mmio_mitigation == MMIO_MITIGATION_OFF ||
>> +	    rfds_mitigation == RFDS_MITIGATION_OFF) {
>> +		if (mds_mitigation == MDS_MITIGATION_OFF &&
>> +		    taa_mitigation == TAA_MITIGATION_OFF &&
>> +		    mmio_mitigation == MMIO_MITIGATION_OFF &&
>> +		    rfds_mitigation == RFDS_MITIGATION_OFF)
>> +			return;
> 
> Ugh, why don't you simply XOR the 4 values and if it's 1 it means the 
> inputs differe => there is inconsistency.
> 
That's certainly cleaner. Will update.

Thx
>> +
>> +		pr_info("MDS, TAA, MMIO Stale Data, and Register File Data Sampling all depend on VERW\n");
>> +		pr_info("In order to disable any one of them please ensure all 4 are disabled.\n");
>> +	}
>> +}
>> +
>>   static void __init md_clear_select_mitigation(void)
>>   {
>> +	verw_mitigations_check();
>>   	mds_select_mitigation();
>>   	taa_select_mitigation();
>>   	mmio_select_mitigation();