Re: [PATCH v4 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

[Sean Christopherson]

On Wed, Mar 31, 2021, Kuppuswamy Sathyanarayanan wrote:
> Changes since v3:
>  * WARN user if SEAM does not disable MONITOR/MWAIT instruction.

Why bother?  There are a whole pile of features that are dictated by the TDX
module spec.  MONITOR/MWAIT is about as uninteresting as it gets, e.g. absolute
worst case scenario is the guest kernel crashes, whereas a lot of spec violations
would compromise the security of the guest.

> +	case EXIT_REASON_MONITOR_INSTRUCTION:
> +	case EXIT_REASON_MWAIT_INSTRUCTION:
> +		/*
> +		 * MWAIT/MONITOR features are disabled by TDX Module (SEAM)
> +		 * and also re-suppressed in kernel by clearing
> +		 * X86_FEATURE_MWAIT CPU feature flag in tdx_early_init(). So
> +		 * if TD guest still executes MWAIT/MONITOR instruction with
> +		 * above suppression, it needs user attention.
> +		 */
> +		WARN(1, "TD Guest used unsupported MWAIT/MONITOR instruction\n");

Why not just WARN_ONCE and call it good?

> +		break;
>  	default:
>  		pr_warn("Unexpected #VE: %d\n", ve->exit_reason);
>  		return -EFAULT;
> -- 
> 2.25.1
>