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

Chao Gao
2024-08-14T00:48:20Z

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. QEMU can report an
error and define a new CPU model that works with the TDX module. Sometimes,
CPUs may drop features; 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?

>
>People may argue that for the new features that have no vmcs control bit
>(usually the new instruction) face the similar issue. Booting a VM with same
>cpu model on a new platform with such new feature leads to the VM actually
>can use the new feature.
>
>However, for the perspective of CPUID, VMM at least can make sure it
>unchanged, though guest can access the feature even when guest CPUID tells no
>such feature. This is virtualization hole. no one like it.