Re: [PATCH 3/4] KVM: nVMX: accurate emulation of MSR_IA32_CR{0,4}_FIXED1

[Paolo Bonzini]

On 23/11/2016 20:16, David Matlack wrote:
> > Oh, I thought userspace would do that!  Doing it in KVM is fine as well,
> > but then do we need to give userspace access to CR{0,4}_FIXED{0,1} at all?
>
> I think it should be safe for userspace to skip restoring CR4_FIXED1,
> since it is 100% generated based on CPUID. But I'd prefer to keep it
> accessible from userspace, for consistency with the other VMX MSRs and
> for flexibility. The auditing should ensure userspace doesn't restore
> a CR4_FIXED1 that is inconsistent with CPUID.

Or would it just allow userspace to put anything into it, even if it's
inconsistent with CPUID, as long as it's consistent with the host?

> Userspace should restore CR0_FIXED1 in case future CPUs change which
> bits of CR0 are valid in VMX operation. Userspace should also restore
> CR{0,4}_FIXED0 so we have the flexibility to change the defaults in
> KVM. Both of these situations seem unlikely but we might as well play
> it safe, the cost is small.

I disagree, there is always a cost.  Besides the fact that it's
unlikely that there'll be any future CR0 bits at all, any changes would
most likely be keyed by a new CPUID bit (the same as CR4) or execution
control (the same as unrestricted guest).

In the end, since we assume that userspace (any) has no idea of what to
do with it, I see no good reason to make the MSRs available.

Paolo