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Date:   Wed, 28 Jun 2023 17:32:13 -0700
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Subject: Re: [PATCH v12 19/22] x86/kexec(): Reset TDX private memory on
 platforms with TDX erratum
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To:     Nikolay Borisov <nik.borisov@suse.com>,
        Kai Huang <kai.huang@intel.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     linux-mm@kvack.org, x86@kernel.org,
        kirill.shutemov@linux.intel.com, tony.luck@intel.com,
        peterz@infradead.org, tglx@linutronix.de, bp@alien8.de,
        mingo@redhat.com, hpa@zytor.com, seanjc@google.com,
        pbonzini@redhat.com, david@redhat.com, dan.j.williams@intel.com,
        rafael.j.wysocki@intel.com, ashok.raj@intel.com,
        reinette.chatre@intel.com, len.brown@intel.com, ak@linux.intel.com,
        isaku.yamahata@intel.com, ying.huang@intel.com, chao.gao@intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com, bagasdotme@gmail.com,
        sagis@google.com, imammedo@redhat.com
References: <cover.1687784645.git.kai.huang@intel.com>
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 <1662a5ef-c333-d6d6-7605-060f4bcca6fd@suse.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <1662a5ef-c333-d6d6-7605-060f4bcca6fd@suse.com>
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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.