Re: [PATCH 6/6] x86/bugs: Clean-up verw mitigations

[Daniel Sneddon]

On 10/7/24 12:37, Josh Poimboeuf wrote:
> On Tue, Sep 24, 2024 at 03:31:40PM -0700, Daniel Sneddon wrote:
>> +static void __init md_clear_select_mitigation(void)
>> +{
>>  	/*
>> -	 * X86_FEATURE_CLEAR_CPU_BUF is now enabled. Update MDS, TAA and MMIO
>> -	 * Stale Data mitigation, if necessary.
>> +	 * If no CPU bug needs VERW, all VERW mitigations are disabled, or all
>> +	 * mitigations are disabled we bail.
>>  	 */
> 
> It's already clear what the code is doing, no comment necessary.
> 
Will remove.
>> -	if (mds_mitigation == MDS_MITIGATION_OFF &&
>> -	    boot_cpu_has_bug(X86_BUG_MDS)) {
>> +	if (!cpu_bug_needs_verw() || verw_mitigations_disabled() ||
>> +	    cpu_mitigations_off()) {
>> +		mds_mitigation = MDS_MITIGATION_OFF;
>> +		taa_mitigation = TAA_MITIGATION_OFF;
>> +		mmio_mitigation = MMIO_MITIGATION_OFF;
>> +		rfds_mitigation = RFDS_MITIGATION_OFF;
>> +		goto out;
>> +	}
> 
> In the case of verw_mitigations_disabled() it's weird to write the
> variables again if they're already OFF.  That should be a separate
> check.
> 
Sure. I will separate them out.
>> +
>> +	/* Check that at least one mitigation is using the verw mitigaiton.
>> +	 * If the cpu doesn't have the correct ucode or if the BUG_* is mitigated
>> +	 * by disabling a feature we won't want to use verw. Ignore MMIO
>> +	 * for now since it depends on what the others choose.
>> +	 */
> 
> Again I think this comment isn't needed as the code is pretty
> straightforward.  The only surprise is the MMIO dependency on
> X86_FEATURE_CLEAR_CPU_BUF, but that's called out below.
> 
Will remove.
>> +
>> +	if (boot_cpu_has_bug(X86_BUG_MDS)) {
>>  		mds_mitigation = MDS_MITIGATION_FULL;
>>  		mds_select_mitigation();
>> +	}  else {
>> +		mds_mitigation = MDS_MITIGATION_OFF;
>>  	}
>> -	if (taa_mitigation == TAA_MITIGATION_OFF &&
>> -	    boot_cpu_has_bug(X86_BUG_TAA)) {
>> +	if (boot_cpu_has_bug(X86_BUG_TAA)) {
>>  		taa_mitigation = TAA_MITIGATION_VERW;
>>  		taa_select_mitigation();
>> +	} else {
>> +		taa_mitigation = TAA_MITIGATION_OFF;
>>  	}
>> -	/*
>> -	 * MMIO_MITIGATION_OFF is not checked here so that mmio_stale_data_clear
>> -	 * gets updated correctly as per X86_FEATURE_CLEAR_CPU_BUF state.
>> -	 */
>> +	if (boot_cpu_has_bug(X86_BUG_RFDS)) {
>> +		rfds_mitigation = RFDS_MITIGATION_VERW;
>> +		rfds_select_mitigation();
>> +	} else {
>> +		rfds_mitigation = RFDS_MITIGATION_OFF;
>> +	}
> 
> This spaghetti can be simplifed by relying on *_select_mitigation() to
> set the mitigation, for example:
> 
> static void __init mds_select_mitigation(void)
> {
> 	if (!boot_cpu_has_bug(X86_BUG_MDS))
> 		mds_mitigation = MDS_MITIGATION_OFF;
> 	else if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
> 		mds_mitigation = MDS_MITIGATION_VERW;
> 	else
> 		mds_mitigation = MDS_MITIGATION_VMWERV;
> }
> 
> Then you can just do:
> 
> 	mds_select_mitigation();
> 	taa_select_mitigation();
> 	rfds_select_mitigation();
> 
> 
You're right. That is much cleaner. Will fix.
>> +	if (mds_mitigation == MDS_MITIGATION_FULL ||
>> +	    taa_mitigation == TAA_MITIGATION_VERW ||
>> +	    rfds_mitigation == RFDS_MITIGATION_VERW)
> 
> For consistency can we rename MDS_MITIGATION_FULL to
> MDS_MITIGATION_VERW?
> 
Will do!
>> +		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
>> +
>> +	/* Now handle MMIO since it may not use X86_FEATURE_CLEAR_CPU_BUF */
> 
> I would clarify this a bit, something like:
> 
> 	/*
> 	 * The MMIO mitigation has a dependency on
> 	 * X86_FEATURE_CLEAR_CPU_BUF so this must be called after it
> 	 * gets set.
> 	 */
> 
Will update.
>>  	if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
>>  		mmio_mitigation = MMIO_MITIGATION_VERW;
>>  		mmio_select_mitigation();
>> +	} else {
>> +		mmio_mitigation = MMIO_MITIGATION_OFF;
>>  	}
>> -	if (rfds_mitigation == RFDS_MITIGATION_OFF &&
>> -	    boot_cpu_has_bug(X86_BUG_RFDS)) {
>> -		rfds_mitigation = RFDS_MITIGATION_VERW;
>> -		rfds_select_mitigation();
>> -	}
>> +
>> +	/* handle nosmt */
> 
> Again I think this comment is superfluous.
> 
Will remove.
>> +	if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
>> +	    (mds_nosmt || cpu_mitigations_auto_nosmt()))
>> +		cpu_smt_disable(false);
>> +
>> +	if (taa_nosmt || mmio_nosmt || cpu_mitigations_auto_nosmt())
>> +		cpu_smt_disable(false);
>> +
> 

Thanks for the review!