[v3,0/2] Fix issue with not starting nesting guests on my system

Message

Maxim Levitsky May 27, 2020, 2:04 p.m. UTC

On my AMD machine I noticed that I can't start any nested guests,
because nested KVM (everything from master git branches) complains
that it can't find msr MSR_IA32_UMWAIT_CONTROL which my system doesn't support
at all anyway.

I traced it to the recently added UMWAIT support to qemu and kvm.
The kvm portion exposed the new MSR in KVM_GET_MSR_INDEX_LIST without
checking that it the underlying feature is supported in CPUID.
It happened to work when non nested because as a precation kvm,
tries to read each MSR on host before adding it to that list,
and when read gets a #GP it ignores it.

When running nested, the L1 hypervisor can be set to ignore unknown
msr read/writes (I need this for some other guests), thus this safety
check doesn't work anymore.

V2: * added a patch to setup correctly the X86_FEATURE_WAITPKG kvm capability
    * dropped the cosmetic fix patch as it is now fixed in kvm/queue

V3: addressed the review feedback and possibly made the commit messages a bit better
    Thanks!

Best regards,
        Maxim Levitsky

Maxim Levitsky (2):
  KVM: VMX: enable X86_FEATURE_WAITPKG in KVM capabilities
  KVM: x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally

 arch/x86/kvm/vmx/vmx.c | 3 +++
 arch/x86/kvm/x86.c     | 4 ++++
 2 files changed, 7 insertions(+)