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: <6e3ae1bd-c6e4-b7cb-ab4c-50849ffbc237@intel.com>
Date:   Tue, 13 Jun 2023 17:24:54 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.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 6/13/23 16:18, Huang, Kai wrote:
> On Tue, 2023-06-13 at 07:25 -0700, Hansen, Dave wrote:
>> On 6/12/23 17:51, Huang, Kai wrote:
>>> If we introduce a helper to mark a page as TDX private page,
>> Let me get this right: you have working, functional code for a
>> highly-unlikely scenario (kernel bugs or even more rare hardware
>> errors).  But, you want to optimize this super-rare case?  It's not fast
>> enough?
>>
>> Is there any other motivation here that I'm missing?
>>
> No it's not about speed.  The motivation is to have a common code to yield less
> line of code, though I don't have clear number of how many LoC can be reduced.

OK, so ... ballpark.  How many lines of code are we going to _save_ for
this super-rare case?  10?  100?  1000?

The upside is saving X lines of code ... somewhere.  The downside is
adding Y lines of code ... somewhere else and maybe breaking things in
the process.

You've evidently done _some_ kind of calculus in your head to make this
tradeoff worthwhile.  I'd love to hear what your calculus is, even if
it's just a gut feel.

Could you share your logic here, please?