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

Dave Hansen
2023-06-12T15:39:30Z

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.