linux-kernel - Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on demand

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Message-ID: <20230630183020.GA4253@hirez.programming.kicks-ass.net>
Date:   Fri, 30 Jun 2023 20:30:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "hpa@...or.com" <hpa@...or.com>, "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "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>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on
 demand

On Fri, Jun 30, 2023 at 09:55:32AM +0000, Huang, Kai wrote:
> On Fri, 2023-06-30 at 11:26 +0200, Peter Zijlstra wrote:
> > On Thu, Jun 29, 2023 at 12:10:00AM +0000, Huang, Kai wrote:
> > > On Wed, 2023-06-28 at 15:17 +0200, Peter Zijlstra wrote:
> > > > On Tue, Jun 27, 2023 at 02:12:37AM +1200, Kai Huang wrote:
> > > > > +EXPORT_SYMBOL_GPL(tdx_cpu_enable);
> > > > 
> > > > I can't find a single caller of this.. why is this exported?
> > > 
> > > It's for KVM TDX patch to use, which isn't in this series.
> > > 
> > > I'll remove the export.  KVM TDX series can export it.
> > 
> > Fair enough; where will the KVM TDX series call this? Earlier there was
> > talk about doing it at kvm module load time -- but I objected (and still
> > do object) to that.
> > 
> > What's the current plan?
> > 
> 
> The direction is still doing it during module load (not my series anyway).  But
> this can be a separate discussion with KVM maintainers involved.

They all on Cc afaict.

> I understand you have concern that you don't want to have the memory & cpu time
> wasted on enabling TDX by default.  For that we can have a kernel command line
> to disable TDX once for all (we can even make it default).

That's insane, I don't want to totally disable it. I want it done at
guard creation. Do the whole TDX setup the moment you actually create a
TDX guast.

Totally killing TDX is stupid, just about as stupid as doing it on
module load (which equates to always doing it).

> Also, KVM will have a module parameter 'enable_tdx'.  I am hoping this could
> reduce your concern too.

I don't get this obsession with doing at module load time :/