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

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Message-ID: <3a3199b09e86cb4699a13f392842736b482831e3.camel@intel.com>
Date:   Mon, 19 Jun 2023 23:35:52 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Hansen, Dave" <dave.hansen@...el.com>
CC:     "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>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "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 Mon, 2023-06-19 at 17:46 +0300, kirill.shutemov@...ux.intel.com wrote:
> On Mon, Jun 19, 2023 at 07:31:21AM -0700, Dave Hansen wrote:
> > On 6/19/23 04:43, Huang, Kai wrote:
> > > On Mon, 2023-06-12 at 06:47 -0700, Dave Hansen wrote:
> > > > On 6/12/23 03:27, Huang, Kai wrote:
> > > > > So I think a __mb() after setting tdmr->pamt_4k_base should be good enough, as
> > > > > it guarantees when setting to any pamt_*_size happens, the valid pamt_4k_base
> > > > > will be seen by other cpus.
> > > > > 
> > > > > Does it make sense?
> > > > Just use a normal old atomic_t or set_bit()/test_bit().  They have
> > > > built-in memory barriers are are less likely to get botched.
> > > Hi Dave,
> > > 
> > > 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.
> 

For this patch I think it's overkill to use spinlock because when the rebooting
cpu is reading this all other cpus have been stopped already, so there's no
concurrent thing here.

However I just recall that the next #MC handler patch can also take advantage of
this too because #MC handler can truly run concurrently with module
initialization.  Currently that one reads tdx_module_status first but again we
may have the same memory order issue.  So having a spinlock makes sense from #MC
handler patch's point of view.

I'll change to use spinlock if Dave is fine?

Thanks for feedback!