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

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Message-ID: <20230628122957.sl7wzenjby3o2xsw@box.shutemov.name>
Date:   Wed, 28 Jun 2023 15:29:57 +0300
From:   kirill.shutemov@...ux.intel.com
To:     Kai Huang <kai.huang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org, dave.hansen@...el.com,
        tony.luck@...el.com, peterz@...radead.org, tglx@...utronix.de,
        bp@...en8.de, mingo@...hat.com, hpa@...or.com, seanjc@...gle.com,
        pbonzini@...hat.com, david@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, ashok.raj@...el.com,
        reinette.chatre@...el.com, len.brown@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, ying.huang@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, nik.borisov@...e.com,
        bagasdotme@...il.com, sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v12 19/22] x86/kexec(): Reset TDX private memory on
 platforms with TDX erratum

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.

Hm?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov