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: <2ddb8858ad995904654379ee57a5949dee3f696b.camel@intel.com>
Date:   Wed, 14 Jun 2023 00:38:36 +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>,
        "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 Tue, 2023-06-13 at 17:24 -0700, Dave Hansen wrote:
> 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?

~50 LoC I guess, certainly < 100.

> 
> 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?

The logic is the whole tdx_is_private_mem() function in the next patch (#MC
handling one) can be significantly reduced from 100 -> ~10, and we roughly needs
some more code (<50 LoC) to mark PAMT as private.