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

[Huang, Kai]

On Wed, 2023-06-28 at 15:04 +0200, Peter Zijlstra wrote:
> On Tue, Jun 27, 2023 at 02:12:37AM +1200, Kai Huang wrote:
> 
> > +static int try_init_module_global(void)
> > +{
> > +	unsigned long flags;
> > +	int ret;
> > +
> > +	/*
> > +	 * The TDX module global initialization only needs to be done
> > +	 * once on any cpu.
> > +	 */
> > +	raw_spin_lock_irqsave(&tdx_global_init_lock, flags);
> > +
> > +	if (tdx_global_initialized) {
> > +		ret = 0;
> > +		goto out;
> > +	}
> > +
> > +	/* All '0's are just unused parameters. */
> > +	ret = seamcall(TDH_SYS_INIT, 0, 0, 0, 0, NULL, NULL);
> > +	if (!ret)
> > +		tdx_global_initialized = true;
> > +out:
> > +	raw_spin_unlock_irqrestore(&tdx_global_init_lock, flags);
> > +
> > +	return ret;
> > +}
> 
> How long does that TDX_SYS_INIT take and why is a raw_spinlock with IRQs
> disabled the right way to serialize this?

The spec says it doesn't have a latency requirement, so theoretically it could
be long.  SEAMCALL is a VMEXIT so it would at least cost thousands of cycles.

If raw_spinlock isn't desired, I think I can introduce another function to do
this and let the caller to call it before calling tdx_cpu_enable().  E.g., we
can have below functions:

1) tdx_global_init()	-> TDH_SYS_INIT
2) tdx_cpu_init()	-> TDH_SYS_LP_INIT
3) tdx_enable()		-> actual module initialization

How does this sound?