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Date:   Wed, 23 Nov 2022 10:18:45 -0800
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Subject: Re: [PATCH v7 06/20] x86/virt/tdx: Shut down TDX module in case of
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To:     Sean Christopherson <seanjc@google.com>
Cc:     "Huang, Kai" <kai.huang@intel.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Luck, Tony" <tony.luck@intel.com>,
        "bagasdotme@gmail.com" <bagasdotme@gmail.com>,
        "ak@linux.intel.com" <ak@linux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Chatre, Reinette" <reinette.chatre@intel.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Yamahata, Isaku" <isaku.yamahata@intel.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        "Shahar, Sagi" <sagis@google.com>,
        "imammedo@redhat.com" <imammedo@redhat.com>,
        "Gao, Chao" <chao.gao@intel.com>,
        "Brown, Len" <len.brown@intel.com>,
        "sathyanarayanan.kuppuswamy@linux.intel.com" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <Y35aXX5b2Ed4vc6y@google.com>
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On 11/23/22 09:37, Sean Christopherson wrote:
> On Wed, Nov 23, 2022, Dave Hansen wrote:
>> There's no way we can guarantee _that_.  For one, the PAMT* allocations
>> can always fail.  I guess we could ask sysadmins to fire up a guest to
>> "prime" things, but that seems a little silly.  Maybe that would work as
>> the initial implementation that we merge, but I suspect our users will
>> demand more determinism, maybe a boot or module parameter.
> Oh, you mean all of TDX initialization?  I thought "initialization" here mean just
> doing tdx_enable().

Yes, but the first call to tdx_enable() does TDH_SYS_INIT and all the
subsequent work to get the module going.

> Yeah, that's not going to be a viable option.  Aside from lacking determinisim,
> it would be all too easy to end up on a system with fragmented memory that can't
> allocate the PAMTs post-boot.

For now, the post-boot runtime PAMT allocations are the one any only way
that TDX can be initialized.  I pushed for it to be done this way.
Here's why:

Doing tdx_enable() is relatively slow and it eats up a non-zero amount
of physically contiguous RAM for metadata (~1/256th or ~0.4% of RAM).
Systems that support TDX but will never run TDX guests should not pay
that cost.

That means that we either make folks opt-in at boot-time or we try to
make a best effort at runtime to do the metadata allocations.

From my perspective, the best-effort stuff is absolutely needed.  Users
are going to forget the command-line opt in and there's no harm in
_trying_ the big allocations even if they fail.

Second, in reality, the "real" systems that can run TDX guests are
probably not going to sit around fragmenting memory for a month before
they run their first guest.  They're going to run one shortly after they
boot when memory isn't fragmented and the best-effort allocation will
work really well.

Third, if anyone *REALLY* cared to make it reliable *and* wanted to sit
around fragmenting memory for a month, they could just start a TDX guest
and kill it to get TDX initialized.  This isn't ideal.  But, to me, it
beats defining some new, separate ABI (or boot/module option) to do it.

So, let's have those discussions.  Long-term, what *is* the most
reliable way to get the TDX module loaded with 100% determinism?  What
new ABI or interfaces are needed?  Also, is that 100% determinism
required the moment this series is merged?  Or, can we work up to it?

I think it can wait until this particular series is farther along.