linux-kernel - Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest

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Date:   Fri, 12 Feb 2021 12:20:11 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest


> On Feb 12, 2021, at 12:06 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> 
> On Fri, Feb 12, 2021, Andy Lutomirski wrote:
>>> On Fri, Feb 5, 2021 at 3:39 PM Kuppuswamy Sathyanarayanan
>>> <sathyanarayanan.kuppuswamy@...ux.intel.com> wrote:
>>> 
>>> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>>> 
>>> The TDX module injects #VE exception to the guest TD in cases of
>>> disallowed instructions, disallowed MSR accesses and subset of CPUID
>>> leaves. Also, it's theoretically possible for CPU to inject #VE
>>> exception on EPT violation, but the TDX module makes sure this does
>>> not happen, as long as all memory used is properly accepted using
>>> TDCALLs.
>> 
>> By my very cursory reading of the TDX arch specification 9.8.2,
>> "Secure" EPT violations don't send #VE.  But the docs are quite
>> unclear, or at least the docs I found are.
> 
> The version I have also states that SUPPRESS_VE is always set.  So either there
> was a change in direction, or the public docs need to be updated.  Lazy accept
> requires a #VE, either from hardware or from the module.  The latter would
> require walking the Secure EPT tables on every EPT violation...
> 
>> What happens if the guest attempts to access a secure GPA that is not
>> ACCEPTed?  For example, suppose the VMM does THH.MEM.PAGE.REMOVE on a secure
>> address and the guest accesses it, via instruction fetch or data access.
>> What happens?
> 
> Well, as currently written in the spec, it will generate an EPT violation and
> the host will have no choice but to kill the guest.

Or page the page back in and try again?

In regular virt guests, if the host pages out a guest page, it’s the host’s job to put it back when needed. In paravirt, a well designed async of protocol can sometimes let the guest to useful work when this happens. If a guest (or bare metal) has its memory hot removed (via balloon or whatever) and the kernel messes up and accesses removed memory, the guest (or bare metal) is toast.

I don’t see why TDX needs to be any different.