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

[stsp]

07.12.2020 14:29, Paolo Bonzini пишет:
> 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
By "that makes sense" you probably
meant to say "bits_that_makes_sense masked
with the ones returned by KVM_GET_SUPPORTED_CPUID"?

So am I right that KVM_SET_CPUID only "lowers"
the supported bits? In which case I don't need to
call it at all, but instead just call KVM_GET_SUPPORTED_CPUID
and see if the needed bits are supported, and
exit otherwise, right?