Re: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup)

[Paolo Bonzini]

On 07/12/20 12:24, stsp wrote:
> It tries to enable VME among other things.
> qemu appears to disable VME by default,
> unless you do "-cpu host". So we have a situation where
> the host (which is qemu) doesn't have VME,
> and guest (dosemu) is trying to enable it.
> Now obviously KVM_SET_CPUID doesn't check anyting
> at all and returns success. That later turns
> into an invalid guest state.
>
>
> Question: should KVM_SET_CPUID check for
> supported bits, end return error if not everything
> is supported?

No, it is intentional.  Most bits of CPUID are not ever checked by KVM, 
so userspace is supposed to set values that makes sense or just copy the 
value of KVM_GET_SUPPORTED_CPUID more or less blindly.

Paolo