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

[Huang, Kai]

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.

Currently for simplicity we just do this inside the module initialization, but
can be optimized later when we have an agreed solution of how to optimize.

> 
> If we have concerns about allocating the PAMT array, can't we use CMA
> for this? Allocate the whole thing at boot as CMA such that when not
> used for TDX it can be used for regular things like userspace and
> filecache pages?

The PAMT allocation itself isn't a concern I think.  The concern is the
TDH.TDMR.INIT to initialize them.

Also, one practical problem to prevent us from pre-allocating PAMT is the PAMT
size to be allocated can only be determined after the TDH.SYS.INFO SEAMCALL,
which reports the "PAMT entry size" in the TDSYSINFO_STRUCT.

> 
> Those TDH.SYS calls should be enough to ensure TDX is actually working,
> no?