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Date:   Tue, 22 Nov 2022 20:13:21 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Kai Huang <kai.huang@intel.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, linux-mm@kvack.org, seanjc@google.com,
        pbonzini@redhat.com, dan.j.williams@intel.com,
        rafael.j.wysocki@intel.com, kirill.shutemov@linux.intel.com,
        ying.huang@intel.com, reinette.chatre@intel.com,
        len.brown@intel.com, tony.luck@intel.com, ak@linux.intel.com,
        isaku.yamahata@intel.com, chao.gao@intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com, bagasdotme@gmail.com,
        sagis@google.com, imammedo@redhat.com
Subject: Re: [PATCH v7 06/20] x86/virt/tdx: Shut down TDX module in case of
 error
Message-ID: <Y30fUS5/JClpBHVc@hirez.programming.kicks-ass.net>
References: <cover.1668988357.git.kai.huang@intel.com>
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On Tue, Nov 22, 2022 at 07:14:14AM -0800, Dave Hansen wrote:
> 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?

Not intialize TDX on busy NOHZ_FULL cpus and hard-limit the cpumask of
all TDX using tasks.

> 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.

Surely modprobe kvm isn't the point where TDX gets loaded? Because
that's on boot for everybody due to all the auto-probing nonsense.

I was expecting TDX to not get initialized until the first TDX using KVM
instance is created. Am I wrong?

> 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.

I'm thikning it makes sense to have a tdx={off,on-demand,force} toggle
anyway.

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

I imagine they don't do that -- in fact I would recommend we make the
whole late loading thing mutually exclusive with nohz_full; can't have
both.