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Date:   Tue, 20 Jun 2023 09:48:05 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        "Huang, Kai" <kai.huang@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
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        "bagasdotme@gmail.com" <bagasdotme@gmail.com>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Chatre, Reinette" <reinette.chatre@intel.com>,
        "Christopherson,, Sean" <seanjc@google.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Yamahata, Isaku" <isaku.yamahata@intel.com>,
        "Shahar, Sagi" <sagis@google.com>,
        "imammedo@redhat.com" <imammedo@redhat.com>,
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        "Brown, Len" <len.brown@intel.com>,
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        "Huang, Ying" <ying.huang@intel.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>
Subject: Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private
 memory during kexec() and reboot
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On Mon, Jun 19, 2023 at 04:41:13PM -0700, Dave Hansen wrote:
> On 6/19/23 07:46, kirill.shutemov@linux.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.

Depends on what you mean; anything that spin-waits will deadlock,
doesn't matter if its a test-and-set or not.

The thing with these non-maskable exceptions/interrupts is that they
must be wait-free. If serialization is required it needs to be try based
and accept failure without waiting.