linux-kernel - Re: [PATCH v3 04/21] x86/virt/tdx: Add skeleton for detecting and initializing TDX on demand

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Date:   Thu, 28 Apr 2022 17:26:52 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 04/21] x86/virt/tdx: Add skeleton for detecting and
 initializing TDX on demand

On 4/28/22 17:11, Kai Huang wrote:
> This is true.  So I think w/o taking the lock is also fine, as the TDX module
> initialization is a state machine.  If any cpu goes offline during logical-cpu
> level initialization and TDH.SYS.LP.INIT isn't done on that cpu, then later the
> TDH.SYS.CONFIG will fail.  Similarly, if any cpu going offline causes
> TDH.SYS.KEY.CONFIG is not done for any package, then TDH.SYS.TDMR.INIT will
> fail.

Right.  The worst-case scenario is someone is mucking around with CPU
hotplug during TDX initialization is that TDX initialization will fail.

We *can* fix some of this at least and provide coherent error messages
with a pattern like this:

	cpus_read_lock();
	// check that all MADT-enumerated CPUs are online
	tdx_init();
	cpus_read_unlock();

That, of course, *does* prevent CPUs from going offline during
tdx_init().  It also provides a nice place for an error message:

	pr_warn("You offlined a CPU then want to use TDX?  Sod off.\n");

> A problem (I realized it exists in current implementation too) is shutting down
> the TDX module, which requires calling TDH.SYS.LP.SHUTDOWN on all BIOS-enabled
> cpus.  Kernel can do this SEAMCALL at most for all present cpus.  However when
> any cpu is offline, this SEAMCALL won't be called on it, and it seems we need to
> add new CPU hotplug callback to call this SEAMCALL when the cpu is online again.

Hold on a sec.  If you call TDH.SYS.LP.SHUTDOWN on any CPU, then TDX
stops working everywhere, right?  But, if someone offlines one CPU, we
don't want TDX to stop working everywhere.