On Fri, Jun 26, 2020 at 01:37:50PM -0400, Peter Xu wrote:
> On Fri, Jun 26, 2020 at 08:56:57AM -0700, Sean Christopherson wrote:
> > Not really? It's solving a problem that doesn't exist in the current code
> > base (assuming TSC_CTRL is fixed), and IMO solving it in an ugly fashion.
> >
> > I would much prefer that, _if_ we want to support blind KVM-internal MSR
> > accesses, we end up with code like:
> >
> > if (msr_info->kvm_internal) {
> > return 1;
> > } else if (!ignore_msrs) {
> > vcpu_debug_ratelimited(vcpu, "unhandled wrmsr: 0x%x data
> > 0x%llx\n",
> > msr, data);
> > return 1;
> > } else {
> > if (report_ignored_msrs)
> > vcpu_unimpl(vcpu,
> > "ignored wrmsr: 0x%x data 0x%llx\n",
> > msr, data);
> > break;
> > }
> >
> > But I'm still not convinced that there is a legimiate scenario for setting
> > kvm_internal=true.
>
> Actually this really looks like my initial version when I was discussing this
> with Paolo before this version, but Paolo suggested what I implemented last.
> I
> think I agree with Paolo that it's an improvement to have a way to get/set
> real
> msr value so that we don't need to further think about effects being taken
> with
> the two tricky msr knobs (report_ignored_msrs, ignore_msrs). These knobs are
> even trickier to me when they're hidden deep, because they are not easily
> expected when seeing the name of the functions (e.g. __kvm_set_msr, rather
> than
> __kvm_set_msr_retval_fixed).
My argument is that it's a KVM bug if we ever encounter do the wrong thing
based on a KVM-internal MSR access. The proposed change would actually make
it _harder_ to find the bug that prompted this patch, as the bogus
__kvm_get_msr() in kvm_cpuid() would silently fail.
If anything, I would argue for something like:
if (WARN_ON_ONCE(msr_info->kvm_internal)) {
return 1;
} else if (!ignore_msrs) {
...
} else {
...
}
I.e. KVM-internal accesses should always pre-validate the existence of the
MSR, if not the validity of the MSR from the guest's perspective.
