Re: [Qemu-devel] [PATCH v3 1/2] i386/kvm: add support for Hyper-V reenlightenment MSRs

[Roman Kagan]

On Tue, Mar 20, 2018 at 03:32:27PM -0300, Eduardo Habkost wrote:
> On Tue, Mar 20, 2018 at 06:34:59PM +0100, Vitaly Kuznetsov wrote:
> > @@ -1747,6 +1760,15 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
> >              if (cpu->hyperv_time) {
> >                  kvm_msr_entry_add(cpu, HV_X64_MSR_REFERENCE_TSC,
> >                                    env->msr_hv_tsc);
> > +
> > +                if (has_msr_hv_reenlightenment) {
> 
> I see that the current code is inconsistent: some entries check
> for has_msr_hv_*, other entries check cpu->hyperv_*.
> 
> I suggest changing all of them (including this one) to check
> cpu->hyperv_* instead.
> 
> The difference between both approaches is that checking just
> has_msr_hv_* would let a non-cooperating guest prevent itself
> from being migrated to an older host by writing a non-zero value
> to a MSR, even if hyperv support was not enabled in the VM
> configuration at all.  I don't think we want that.

Agreed.  We accumulated a number of these over time; it's mostly my
fault, so I don't feel it's just to ask Vitaly to fix the existing ones,
but let's not add new ones.  

Roman.