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Subject: Re: [RFC v1 00/26] Add TDX Guest Support
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        linux-kernel@vger.kernel.org
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Message-ID: <4270ec8f-7055-8562-cdb4-aa8cff10269a@intel.com>
Date:   Sat, 3 Apr 2021 09:26:24 -0700
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Precedence: bulk

On 4/2/21 2:32 PM, Andi Kleen wrote:
>> If we go this route, what are the rules and restrictions?  Do we have to
>> say "no MMIO in #VE"?
> 
> All we have to say is "No MMIO in #VE before getting thd TDVEINFO arguments"
> After that it can nest without problems.

Well, not exactly.  You still can't do things that will could cause a n
unbounded recusive #VE.

It doesn't seem *that* far fetched to think that someone might try to
defer some work or dump data to the console.

> If you nest before that the TDX will cause a triple fault.
> 
> The code that cannot do it is a few lines in the early handler which
> runs with interrupts off.

>> Which brings up another related point: How do you debug TD guests?  Does
>> earlyprintk work?
> 
> Today it works actually because serial ports are allowed. But I expect it to
> be closed eventually because serial code is a lot of code to audit. 
> But you can always disable the filtering with a command line option and
> then it will always work for debugging.

Do we need a TDX-specific earlyprintk?  I would imagine it's pretty easy
to implement.