Re: [kvm-unit-tests PATCH] x86: Fix max VMCS field encoding index check

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, Dec 13, 2019 at 09:30:45AM -0800, Jim Mattson wrote:
> On Fri, Dec 13, 2019 at 1:13 AM Nadav Amit <nadav.amit@gmail.com> wrote:
> >
> > > On Dec 13, 2019, at 12:59 AM, Jim Mattson <jmattson@google.com> wrote:
> > >
> > > On Sat, May 18, 2019 at 4:58 PM Nadav Amit <nadav.amit@gmail.com> wrote:
> > >> The test that checks the maximum VMCS field encoding does not probe all
> > >> possible VMCS fields. As a result it might fail since the actual
> > >> IA32_VMX_VMCS_ENUM.MAX_INDEX would be higher than the expected value.
> > >>
> > >> Change the test to check that the maximum of the supported probed
> > >> VMCS fields is lower/equal than the actual reported
> > >> IA32_VMX_VMCS_ENUM.MAX_INDEX.
> > >
> > > Wouldn't it be better to probe all possible VMCS fields and keep the
> > > test for equality?
> >
> > It might take a while though…
> >
> > How about probing VMREAD/VMWRITE to MAX_INDEX in addition to all the known
> > VMCS fields and then checking for equation?
> >
> It can't take that long. VMCS field encodings are only 15 bits, and
> you can ignore the "high" part of 64-bit fields, so that leaves only
> 14 bits.

Unless kvm-unit-tests is being run in L1, in which case things like this
are painful.  That being said, I do agree that probing "all" VMCS fields
is the way to go.  Walking from highest->lowest probably won't even take
all that many VMREADS.  If it is slow, the test can be binned to its own
config.