Re: [EXTERNAL] Re: [PATCH 4/4] kvm: svm: Support KVM_SEV_SNP_PAGE_TYPE_VMSA at SNP_LAUNCH_UPDATE

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On 2026-06-24 00:02, Sean Christopherson wrote:
> On Tue, Jun 23, 2026, Jethro Beekman wrote:
>> On 2026-06-23 23:43, Sean Christopherson wrote:
>>> On Tue, Jun 23, 2026, Jethro Beekman wrote:
>>>> On 2026-06-23 16:51, Jon Lange wrote:
>>>>> On Tuesday, June 23, 2026 6:40 AM, Sean Christopherson wrote:
>>>>>> On Wed, Jun 17, 2026, J=C3=B6rg R=C3=B6del wrote:
>>>>>>> On Wed, Jun 17, 2026 at 06:37:52AM -0700, Sean Christopherson wro=
te:
>>>>>>>> Ok, so it took us a few times to learn our lesson.  I still don'=
t see that as a
>>>>>>>> strong argument for new uAPI, especially not for VMSA pages.  I =
am very firmly
>>>>>>>> of the opinion that letting anything but the host kernel configu=
re the VMSA is
>>>>>>>> beyond stupid, but unfortunately we're stuck with AP_CREATION.  =
Expanding that
>>>>>>>> surface has a very, very, VERY high bar to get over.
>>>>>>>
>>>>>>> The strongest argument in my view (and the main reason we are doi=
ng this) is
>>>>>>> actually the predictable launch measurement. On SEV-SNP this is a=
 requirement
>>>>>>> to use platform VM-identity features like the ID Block.
>>>>>>
>>>>>> And I'm saying that unless KVM *can't* provide a predictable launc=
h measurement,
>>>>>> which AIUI isn't the case, then the launch measurement *must* be s=
table across
>>>>>> kernels because it's part of KVM's ABI.  So as I see it, the issue=
 isn't that
>>>>>> KVM is inherently unpredictable, it's that we lack tests to valida=
te a thorny,
>>>>>> subtle piece of KVM's ABI.
>>>>>
>>>>> Joerg is suggesting that we need a launch measurement that is stabl=
e not
>>>>> just across multiple launches on the same system, but across multip=
le
>>>>> hypervisors.
>>>>
>>>> If this is now suddenly an acceptable argument, please also merge ht=
tps://lkml.org/lkml/2021/4/12/625
>>>
>>> No, that's not how this works.  Dave and Jarkko both have unanswered =
questions
>>> regarding the use case.  Answer their questions, and I'm confident yo=
u'll get
>>> traction.
>>>
>>>  : I don't believe we necessarily *WANT* or need Linux to support "al=
l
>>>  : possible ECREATE, EADD, EEXTEND, EINIT sequences".  Yet, it's what=
 is
>>>  : being used to justify this series without any other justification.=

>>>  :=20
>>>  : It's going to be a different story if you bring me a real enclave =
that
>>>  : *REALLY* wants to do this for good reasons.
>>>
>>> and
>>>
>>>  : What specifically are you referring as the "rest of the world"?
>>>  :=20
>>>  : That would be mean that there is reviewable workload "out there".
>>>
>>> The start of that thread is *exactly* what's playing out here.  The b=
ig difference,
>>> and why this one's likely getting a different result, is that Jon pro=
vided a very
>>> thorough explanation of exactly what use case Joerg and company want =
to support.
>>> The only "assumed knowledge" is why it's desirable for the measuremen=
t to be stable
>>> across hypervisors, but I'm comfortable stitching that together on my=
 own.
>>>
>>> In other words, they aren't asking KVM to support every possible way/=
time a VMSA
>>> could be associated with a vCPU, they're asking to extend KVM to supp=
ort a concrete
>>> use case, with meaningful real world impact.  In fact, I actually thi=
nk this series
>>> is *too* narrowly focused on their use case.
>>>
>>> FWIW, KVM_TDX_INIT_MEM_REGION has almost the exact same ABI that SGX =
has: pages
>>> contents are measured immediately after they're added.
>>
>> Sorry, the argument from Jon/Joerg is no different than the one we mad=
e years
>> ago for SGX.
>>
>> It was previously made abundantly clear that it's the kernel maintaine=
rs's
>> stance that it's sufficient to offer a UAPI that allows the creation o=
f a
>> Linux-specific subset of enclaves with a stable measurement encompassi=
ng
>> largely the enclave functionality available in the hardware. Or said a=
nother
>> way: if you can take your enclave and modify it to work with Linux, al=
beit
>> with a different measurement, that is acceptable. KVM-SNP is currently=
 at
>> that state: it is currently completely possible to load VMs that were
>> designed with the current Linux VMSA in IGVM format.
>>
>> A UAPI that allows creation of all architecturally-valid enclaves for
>> purposes of measurement portability was stated as an explicit non-goal=
=2E As
>> this is the only stated purpose of this patch set, it should also not =
be
>> accepted.
>=20
> No, the stated goals (thanks to the follow-up from Jon) are to (a) supp=
ort VMSA
> GPAs other than KVM's hardcoded, arbitrary 0xFFFFFFFFF000, and=20

As Jon points out, the VMSA GPA is not architecturally relevant, so any e=
nclave can be changed to support a BSP VMSA at 0xFFFFFFFFF000. The only r=
eason to "support VMSA GPAs other than KVM's hardcoded, arbitrary 0xFFFFF=
FFFF000" is measurement portability.

> (b) to play nice with multi-VMPL scenarios in the future.

It's not clear to me what multi-VMPL scenario Jon is describing that requ=
ires custom VMSA content/address at launch time other than measurement po=
rtability. It's currently completely possible for VMPL0 code to set the V=
MPL of a VMSA or create a new VMSA with VMPL>0 at any GPA of their choosi=
ng.

> If that happens to let userspace build
> every architecturally possible VM, then yay!, we got lucky and probably=
 won't
> ever need to revisit this.  But "support every possibility" is not the =
goal.

--=20
Jethro Beekman | CTO | Fortanix


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