Re: [PATCH 0/4] KVM: nVMX: Fixes for VMX capability MSR invariance

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 02, 2022, Jim Mattson wrote:
> On Wed, Feb 2, 2022 at 4:33 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Wed, Feb 02, 2022, Jim Mattson wrote:
> > > On Wed, Feb 2, 2022 at 3:04 PM Oliver Upton <oupton@google.com> wrote:
> > > >
> > > > Ultimately, it is the responsibility of userspace to configure an
> > > > appropriate MSR value for the CPUID it provides its guest. However,
> > > > there are a few bits in VMX capability MSRs where KVM intervenes. The
> > > > "load IA32_PERF_GLOBAL_CTRL", "load IA32_BNDCFGS", and "clear
> > > > IA32_BNDCFGS" bits in the VMX VM-{Entry,Exit} control capability MSRs
> > > > are updated every time userspace sets the guest's CPUID. In so doing,
> > > > there is an imposed ordering between ioctls, that userspace must set MSR
> > > > values *after* setting the guest's CPUID.
> > >
> > >  Do you mean *before*?
> >
> > No, after, otherwise the CPUID updates will override the MSR updates.
> 
> Wasn't that the intention behind this code in the first place (to
> override KVM_SET_MSR based on CPUID bits)? If not, what was the
> intention behind this code?

The MPX side is from commit 5f76f6f5ff96 ("KVM: nVMX: Do not expose MPX VMX controls
when guest MPX disabled"), which was a miguided "fix" to workaround an L1 KVM bug in L0.
And also to workaround an L0 userspace bug (failure to set VMX MSRs).

The PMU bug looks to be a similar snafu, it appears to workaround a userspace bug
(again, failure to set VMX MSRs) in KVM.

But once userspace started taking ownership of VMX MSRs, KVM's hack-a-fixes just got
in the way :-/