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

Maxim Levitsky
2020-05-23T16:15:10Z

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

Best regards,
Maxim Levitsky

Maxim Levitsky (2):
kvm/x86/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(+)

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2.26.2