linux-kernel - Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot

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Date:   Mon, 19 Jun 2023 16:41:13 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Cc:     "Huang, Kai" <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "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>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private
 memory during kexec() and reboot

On 6/19/23 07:46, kirill.shutemov@...ux.intel.com wrote:
>>>
>>> Using atomic_set() requires changing tdmr->pamt_4k_base to atomic_t, which is a
>>> little bit silly or overkill IMHO.  Looking at the code, it seems
>>> arch_atomic_set() simply uses __WRITE_ONCE():
>> How about _adding_ a variable that protects tdmr->pamt_4k_base?
>> Wouldn't that be more straightforward than mucking around with existing
>> types?
> What's wrong with simple global spinlock that protects all tdmr->pamt_*?
> It is much easier to follow than a custom serialization scheme.

Quick, what prevents a:

	spin_lock() => #MC => spin_lock()

deadlock?

Plain old test/sets don't deadlock ever.