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

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Message-ID: <f04c7b69fe539a8065c24f766455c0bec3cdf877.camel@intel.com>
Date:   Thu, 29 Jun 2023 00:27:50 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
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        "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 15:29 +0300, kirill.shutemov@...ux.intel.com wrote:
> On Tue, Jun 27, 2023 at 02:12:49AM +1200, Kai Huang wrote:
> > @@ -1113,6 +1115,17 @@ static int init_tdx_module(void)
> >  	 */
> >  	wbinvd_on_all_cpus();
> >  
> > +	/*
> > +	 * 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.
> > +	 */
> > +	atomic_inc_return(&tdx_may_has_private_mem);
> 
> Why do we need atomics at all here? Writers seems serialized with
> tdx_module_lock and reader accesses the variable when all CPUs, but one is
> down and cannot race.
> 

In kexec() the reader reads this when all remote cpus are dead w/o holding
module lock.  All remote cpus can be stopped at _ANY_ time, meaning they can be
stopped right in any place in middle of init_tdx_module().  IIUC the module lock
doesn't help here.