Re: [PATCH 0/3] AMD invpcid exception fix

[Bandan Das <bsd@redhat.com>]

Jim Mattson <jmattson@google.com> writes:

> On Fri, Feb 12, 2021 at 6:49 AM Bandan Das <bsd@redhat.com> wrote:
>>
>> Paolo Bonzini <pbonzini@redhat.com> writes:
>>
>> > On 11/02/21 22:22, Bandan Das wrote:
>> >> The pcid-disabled test from kvm-unit-tests fails on a Milan host because the
>> >> processor injects a #GP while the test expects #UD. While setting the intercept
>> >> when the guest has it disabled seemed like the obvious thing to do, Babu Moger (AMD)
>> >> pointed me to an earlier discussion here - https://lkml.org/lkml/2020/6/11/949
>> >>
>> >> Jim points out there that  #GP has precedence over the intercept bit when invpcid is
>> >> called with CPL > 0 and so even if we intercept invpcid, the guest would end up with getting
>> >> and "incorrect" exception. To inject the right exception, I created an entry for the instruction
>> >> in the emulator to decode it successfully and then inject a UD instead of a GP when
>> >> the guest has it disabled.
>> >>
>> >> Bandan Das (3):
>> >>    KVM: Add a stub for invpcid in the emulator table
>> >>    KVM: SVM: Handle invpcid during gp interception
>> >>    KVM: SVM:  check if we need to track GP intercept for invpcid
>> >>
>> >>   arch/x86/kvm/emulate.c |  3 ++-
>> >>   arch/x86/kvm/svm/svm.c | 22 +++++++++++++++++++++-
>> >>   2 files changed, 23 insertions(+), 2 deletions(-)
>> >>
>> >
>> > Isn't this the same thing that "[PATCH 1/3] KVM: SVM: Intercept
>> > INVPCID when it's disabled to inject #UD" also does?
>> >
>> Yeah, Babu pointed me to Sean's series after I posted mine.
>> 1/3 indeed will fix the kvm-unit-test failure. IIUC, It doesn't look like it
>> handles the case for the guest executing invpcid at CPL > 0 when it's
>> disabled for the guest - #GP takes precedence over intercepts and will
>> be incorrectly injected instead of an #UD.
>
> I know I was the one to complain about the #GP, but...
>
> As a general rule, kvm cannot always guarantee a #UD for an
> instruction that is hidden from the guest. Consider, for example,
> popcnt, aesenc, vzeroall, movbe, addcx, clwb, ...
> I'm pretty sure that Paolo has brought this up in the past when I've
> made similar complaints.

Ofcourse, even for vm instructions failures, the fixup table always jumps
to a ud2. I was just trying to address the concern because it is possible
to inject the correct exception via decoding the instruction.

Bandan