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Date:   Wed, 5 Jul 2023 09:16:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Huang, Kai" <kai.huang@intel.com>
Cc:     "Christopherson,, Sean" <seanjc@google.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
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Subject: Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on
 demand
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On Tue, Jul 04, 2023 at 09:50:22PM +0000, Huang, Kai wrote:
> On Tue, 2023-07-04 at 18:58 +0200, Peter Zijlstra wrote:
> > On Fri, Jun 30, 2023 at 02:24:56PM -0700, Sean Christopherson wrote:
> > 
> > > Waiting until userspace attempts to create the first TDX guest adds complexity
> > > and limits what KVM can do to harden itself.  Currently, all feature support in
> > > KVM is effectively frozen at module load.  E.g. most of the setup code is
> > > contained in __init functions, many module-scoped variables are effectively 
> > > RO after init (though they can't be marked as such until we smush kvm-intel.ko
> > > and kvm-amd.ko into kvm.ko, which is tentatively the long-term plan).  All of
> > > those patterns would get tossed aside if KVM waits until userspace attempts to
> > > create the first guest.
> > 
> > ....
> > 
> > People got poked and the following was suggested:
> > 
> > On boot do:
> > 
> >  TDH.SYS.INIT
> >  TDH.SYS.LP.INIT
> >  TDH.SYS.CONFIG
> >  TDH.SYS.KEY.CONFIG
> > 
> > This should get TDX mostly sorted, but doesn't consume much resources.
> > Then later, when starting the first TDX guest, do the whole
> > 
> >  TDH.TDMR.INIT
> > 
> > dance to set up the PAMT array -- which is what gobbles up memory. From
> > what I understand the TDH.TDMR.INIT thing is not one of those
> > excessively long calls.
> 
> The TDH.TDMR.INIT itself has it's own latency requirement implemented in the TDX
> module, thus it only initializes a small chunk  (1M I guess) in each call. 
> Therefore we need a loop to do bunch of TDH.TDMR.INIT in order to initialize all
> PAMT entries for all TDX-usable memory, which can be time-consuming.

Yeah, so you can put a cond_resched() in that loop and all is well, you
do not negatively affect other tasks. Because *that* was the concern
raised.