linux-kernel - Re: [PATCH v1 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

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Message-ID: <840efd72-65b1-385c-e9a6-343410c85221@intel.com>
Date:   Tue, 30 Mar 2021 08:10:27 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>, Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

On 3/30/21 8:00 AM, Andi Kleen wrote:
>>> +	/* MWAIT is not supported in TDX platform, so suppress it */
>>> +	setup_clear_cpu_cap(X86_FEATURE_MWAIT);
>> In fact, MWAIT bit returned by CPUID instruction is zero for TD guest. This
>> is enforced by SEAM module.
> Good point.
>> Do we still need to safeguard it by setup_clear_cpu_cap() here?
> I guess it doesn't hurt to do it explicitly.

If this MWAIT behavior (clearing the CPUID bit) is part of the guest
architecture, then it would also be appropriate to WARN() rather than
silently clearing the X86_FEATURE bit.