linux-kernel - Re: [PATCH v12 19/22] x86/kexec(): Reset TDX private memory on platforms with TDX erratum

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Date:   Thu, 29 Jun 2023 00:58:44 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        "Luck, Tony" <tony.luck@...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>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "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>,
        "x86@...nel.org" <x86@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v12 19/22] x86/kexec(): Reset TDX private memory on
 platforms with TDX erratum

On Wed, 2023-06-28 at 17:32 -0700, Dave Hansen wrote:
> On 6/28/23 02:20, Nikolay Borisov wrote:
> > > 
> > >   +    /*
> > > +     * Starting from this point the system may have TDX private
> > > +     * memory.  Make it globally visible so tdx_reset_memory() only
> > > +     * reads TDMRs/PAMTs when they are stable.
> > > +     *
> > > +     * Note using atomic_inc_return() to provide the explicit memory
> > > +     * ordering isn't mandatory here as the WBINVD above already
> > > +     * does that.  Compiler barrier isn't needed here either.
> > > +     */
> > 
> > If it's not needed, then why use it? Simply do atomic_inc() and instead
> > rephrase the comment to state what are the ordering guarantees and how
> > they are achieved (i.e by using wbinvd above).
> 
> Even better, explain why the barrier needs to be there and *IGNORE* the
> WBVIND.
> 
> If the WBINVD gets moved -- or if the gods ever bless us with a halfway
> reasonable way to flush the caches that's not full serializing -- this
> code is screwed.
> 
> There is _zero_ reason to try and "optimize" this junk by trying to get
> rid of a memory barrier at the risk of screwing it over later.
> 
> I use "optimize" in quotes because that's a highly charitable way of
> describing this activity.
> 

Agreed.  I'll try to explain this well and come back.

Thanks!