Re: [PATCH v7 06/20] x86/virt/tdx: Shut down TDX module in case of error

[Dave Hansen]

On 11/22/22 01:13, Peter Zijlstra wrote:
> On Mon, Nov 21, 2022 at 01:26:28PM +1300, Kai Huang wrote:
>> +/*
>> + * Call the SEAMCALL on all online CPUs concurrently.  Caller to check
>> + * @sc->err to determine whether any SEAMCALL failed on any cpu.
>> + */
>> +static void seamcall_on_each_cpu(struct seamcall_ctx *sc)
>> +{
>> +	on_each_cpu(seamcall_smp_call_function, sc, true);
>> +}
> 
> Suppose the user has NOHZ_FULL configured, and is already running
> userspace that will terminate on interrupt (this is desired feature for
> NOHZ_FULL), guess how happy they'll be if someone, on another parition,
> manages to tickle this TDX gunk?

Yeah, they'll be none too happy.

But, what do we do?

There are technical solutions like detecting if NOHZ_FULL is in play and
refusing to initialize TDX.  There are also non-technical solutions like
telling folks in the documentation that they better modprobe kvm early
if they want to do TDX, or their NOHZ_FULL apps will pay.

We could also force the TDX module to be loaded early in boot before
NOHZ_FULL is in play, but that would waste memory on TDX metadata even
if TDX is never used.

How do NOHZ_FULL folks deal with late microcode updates, for example?
Those are roughly equally disruptive to all CPUs.