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

[Nikolay Borisov <nik.borisov@suse.com>]

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@linux.intel.com>
> ---
>   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.

> +
> +		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();