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Date:   Tue, 21 Mar 2023 21:01:17 -0700
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Subject: Re: [PATCH v5 22/34] x86/fred: FRED initialization code
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To:     "Li, Xin3" <xin3.li@intel.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>
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        "Christopherson,, Sean" <seanjc@google.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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On 3/21/23 19:22, Li, Xin3 wrote:
>> If there is no other concrete reason other than overflowing for assigning NMI and
>> #DB with a stack level > 0, #VE should also be assigned with a stack level > 0, and
>> #BP too. #VE can happen anytime and anywhere, so it is subject to overflowing too.
> With IDT, both #VE and #BP do not use IST, but NMI, #DB, #MC and #DF do.
> 
> Let's keep this "secret" logic for now, i.e., not change the stack levels
> for #VE and #BP at this point. We can do "optimization", i.e., change them
> later ????.

#VE also can't happen anywhere.  There is some documentation about it in
here:

	https://docs.kernel.org/x86/tdx.html#linux-ve-handler

But, basically, the only halfway sane thing a guest might do to hit a
#VE is touch some "MMIO".  The host can *not* cause them in arbitrary
places because of the SEPT_VE_DISABLE attribute.

#VE's also can't nest until after the guest retrieves the "VE info".
That means that the #VE handler at _least_ reaches C code before it's
subject to another #VE and that second one would still need to be
induced by something the guest does explicitly.