Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 31 Mar 2021 21:53:14 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD
Message-ID: <YGTvSvr2T2v3t3XA@google.com>
References: <2FE32855-EA5D-44E4-AACC-25E9B1476547@amacapital.net>
 <e62cfd0ae90de435e6819979d9027f76d835a22a.1617224710.git.sathyanarayanan.kuppuswamy@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <e62cfd0ae90de435e6819979d9027f76d835a22a.1617224710.git.sathyanarayanan.kuppuswamy@linux.intel.com>
Precedence: bulk

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
>