Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from KVM_GET_SUPPORTED_CPUID

[Chao Gao]

On Tue, Aug 13, 2024 at 06:16:10PM -0700, Sean Christopherson wrote:
>On Wed, Aug 14, 2024, Chao Gao wrote:
>> On Tue, Aug 13, 2024 at 11:14:31PM +0800, Xiaoyao Li wrote:
>> >On 8/13/2024 7:34 PM, Chao Gao wrote:
>> >> I think adding new fixed-1 bits is fine as long as they don't break KVM, i.e.,
>> >> KVM shouldn't need to take any action for the new fixed-1 bits, like
>> >> saving/restoring more host CPU states across TD-enter/exit or emulating
>> >> CPUID/MSR accesses from guests
>> >
>> >I disagree. Adding new fixed-1 bits in a newer TDX module can lead to a
>> >different TD with same cpu model.
>> 
>> The new TDX module simply doesn't support old CPU models.
>
>What happens if the new TDX module is needed to fix a security issue?  Or if a
>customer wants to support a heterogenous migration pool, and older (physical)
>CPUs don't support the feature?  Or if a customer wants to continue hosting
>existing VM shapes on newer hardware?
>
>> QEMU can report an error and define a new CPU model that works with the TDX
>> module. Sometimes, CPUs may drop features;
>
>Very, very rarely.  And when it does happen, there are years of warning before
>the features are dropped.
>
>> this may cause KVM to not support some features and in turn some old CPU
>> models having those features cannot be supported.  is it a requirement for
>> TDX modules alone that old CPU models must always be supported?
>
>Not a hard requirement, but a pretty firm one.  There needs to be sane, reasonable
>behavior, or we're going to have problems.

OK. So, the expectation is the TDX module should avoid adding new fixed-1 bits.

I suppose this also applies to "native" CPUID bits, which are not configurable
and simply reflected as native values to TDs.

One scenario where "fixed-1" bits can help is: we discover a security issue and
release a microcode update to expose a feature indicating which CPUs are
vulnerable. if the TDX module allows the VMM to configure the feature as 0
(i.e., not vulnerable) on vulnerable CPUs, a TD might incorrectly assume it's
not vulnerable, creating a security issue.

I think in above case, the TDX module has to add a "fixed-1" bit. An example of
such a feature is RRSBA in the IA32_ARCH_CAPABILITIES MSR.