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

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Date:   Thu, 29 Jun 2023 00:00:44 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
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        "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 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?