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

[Dave Hansen]

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.