linux-kernel - Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot

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Date:   Tue, 20 Jun 2023 12:56:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private
 memory during kexec() and reboot

On Tue, Jun 20, 2023 at 10:42:32AM +0000, Huang, Kai wrote:
> On Tue, 2023-06-20 at 10:11 +0200, Peter Zijlstra wrote:
> > On Mon, Jun 12, 2023 at 03:06:48AM +0000, Huang, Kai wrote:
> > 
> > > +       __mb();
> > 
> > __mb() is not a valid interface to use.
> 
> Thanks for feedback!
> 
> May I ask why, for education purpose? :)

it's the raw MFENCE wrapper, not one of the *many* documented barriers.

Also, typicaly you do *not* want MFENCE, MFENCE bad.