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

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2023-06-30 at 11:25 +0200, Peter Zijlstra wrote:
> On Thu, Jun 29, 2023 at 12:00:44AM +0000, Huang, Kai wrote:
> 
> > 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?
> 
> Ah, wait, I hadn't had enough wake-up juice, it's tdx_global_init() that
> did the raw_spinlock_t, but that isn't the IPI thing.
> 
> Then perhaps just use a mutex to serialize things?
> 

In the current code yes TDH_SYS_INIT is protected by raw_spinlock_t, because it
is done in tdx_cpu_enable().  I thought this makes the caller (KVM)'s life
easier as it doesn't have to call an additional tdx_global_init().

If we put TDH_SYS_INIT to an additional tdx_global_init(), then we are
essentially asking the caller to guarantee it must be called before calling any
tdx_cpu_enable() (or tdx_cpu_init() for better naming).  But in this case we
don't need the raw_spinlock anymore because it's caller's responsibility now.

They both are not protected by the TDX module initialization mutex, only
tdx_enable() is.  The caller (KVM) is supposed to call tdx_cpu_enable() for all
online cpus via IPI function call before calling tdx_enable().

So if using raw_spinlock_t around TDH_SYS_INIT is a concern, then we can go with
the dedicated tdx_global_init() function option.

Hope I've explained this clearly.