Re: [PATCH RFC v9 08/51] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

[Dave Hansen]

On 6/11/23 21:25, Michael Roth wrote:
> A hardware limitation prevents the host from enabling Automatic IBRS
> when SNP is enabled.  Instead, fall back to retpolines.

"Hardware limitation"?  As in, it is a documented, architectural
restriction?  Or, it's a CPU bug?

> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index f9d060e71c3e..3fba3623ff64 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1507,7 +1507,12 @@ static void __init spectre_v2_select_mitigation(void)
>  
>  	if (spectre_v2_in_ibrs_mode(mode)) {
>  		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> -			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
> +			if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP)) {
> +				msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
> +			} else {
> +				pr_err("SNP feature available, not enabling AutoIBRS on the host.\n");
> +				mode = spectre_v2_select_retpoline();
> +			}

I think this would be nicer if you did something like:

	if (cpu_feature_enabled(X86_FEATURE_SEV_SNP))
		setup_clear_cpu_cap(X86_FEATURE_AUTOIBRS);

somewhere _else_ in the code instead of smack-dab in the middle of the
mitigation selection.