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Message-ID: <52b2be9b-defd-63ce-4cb2-96cd624a95a6@intel.com>
Date:   Tue, 22 Nov 2022 07:14:14 -0800
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Subject: Re: [PATCH v7 06/20] x86/virt/tdx: Shut down TDX module in case of
 error
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To:     Peter Zijlstra <peterz@infradead.org>,
        Kai Huang <kai.huang@intel.com>
Cc:     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
References: <cover.1668988357.git.kai.huang@intel.com>
 <48505089b645019a734d85c2c29f3c8ae2dbd6bd.1668988357.git.kai.huang@intel.com>
 <Y3ySxEr64HkUaEDq@hirez.programming.kicks-ass.net>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <Y3ySxEr64HkUaEDq@hirez.programming.kicks-ass.net>
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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.