Re: [PATCH v19 130/130] RFC: KVM: x86, TDX: Add check for KVM_SET_CPUID2

[Edgecombe, Rick P]

On Mon, 2024-03-25 at 22:31 +0000, Huang, Kai wrote:
> (sorry replying from outlook due to some issue to my linux box environment)
> 
> It booted because Qemu does sane thing, i.e., it always passes the correct CPUIDs in
> KVM_SET_CPUID2.
> 
> Per-Sean's comments, KVM should guarantee the consistency between CPUIDs done in TDH.MNG.INIT and
> KVM_SET_CPUID2, otherwise if Qemu passes in-consistent CPUIDs KVM can easily fail to work with TD.
> 
> To guarantee the consistency, KVM could do two options as we discussed:
> 
> 1) reject KVM_SET_CPUID2 completely.
> 2) Still allow KVM_SET_CPUID2 but manually check the CPUID consistency between the one done in
> TDH.MNG.INIT and the one passed in KVM_SET_CPUID2.
> 
> 1) can obviously guarantee consistency.  But KVM maintains CPUIDs in 'vcpu', so to make the
> existing KVM code continue to work, we need to manually set 'vcpu->cpuid' to the one that is done
> in TDH.MNG.INIT. 
> 
> 2) you need to check the consistency and reject KVM_SET_CPUID2 if in-consistency found.  But other
> than that, KVM doesn't need to anything more because if we allow KVM_SET_CPUID2, the 'vcpu' will
> have its own CPUIDs populated anyway.

Ah, thanks for explaining. So 1 is not that simple, it is a maybe slightly smaller separate
solution. Now I see why the discussion was to just do the consistency checking up front.