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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest
Date:   Fri, 12 Feb 2021 12:20:11 -0800
Message-Id: <A06CFC3D-53A8-45C1-9580-8459585F458E@amacapital.net>
References: <YCbfyde9jl7ti0Oz@google.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        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>,
        LKML <linux-kernel@vger.kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>
In-Reply-To: <YCbfyde9jl7ti0Oz@google.com>
To:     Sean Christopherson <seanjc@google.com>
Precedence: bulk


> On Feb 12, 2021, at 12:06 PM, Sean Christopherson <seanjc@google.com> wrot=
e:
>=20
> =EF=BB=BFOn Fri, Feb 12, 2021, Andy Lutomirski wrote:
>>> On Fri, Feb 5, 2021 at 3:39 PM Kuppuswamy Sathyanarayanan
>>> <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>>>=20
>>> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>>>=20
>>> 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.
>>=20
>> 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.
>=20
> The version I have also states that SUPPRESS_VE is always set.  So either t=
here
> was a change in direction, or the public docs need to be updated.  Lazy ac=
cept
> requires a #VE, either from hardware or from the module.  The latter would=

> require walking the Secure EPT tables on every EPT violation...
>=20
>> 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 sec=
ure
>> address and the guest accesses it, via instruction fetch or data access.
>> What happens?
>=20
> Well, as currently written in the spec, it will generate an EPT violation a=
nd
> 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=E2=80=99s the=
 host=E2=80=99s 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 happ=
ens. If a guest (or bare metal) has its memory hot removed (via balloon or w=
hatever) and the kernel messes up and accesses removed memory, the guest (or=
 bare metal) is toast.

I don=E2=80=99t see why TDX needs to be any different.=