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Date:   Tue, 26 May 2020 18:03:32 -0700
From:   Krish Sadhukhan <>
To:     Maxim Levitsky <>,
Cc:     Paolo Bonzini <>,
        "H. Peter Anvin" <>, Tao Xu <>,
        Sean Christopherson <>,
        Jim Mattson <>,, Joerg Roedel <>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <>,
        Wanpeng Li <>,
        Ingo Molnar <>,
        Thomas Gleixner <>,
        Borislav Petkov <>,
        Vitaly Kuznetsov <>,
        Jingqi Liu <>
Subject: Re: [PATCH 0/2] Fix issue with not starting nesting guests on my

On 5/23/20 9:14 AM, Maxim Levitsky wrote:
> 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(+)
Nit: The added 'break' statement in patch# 2 is not required.

Reviewed-by: Krish Sadhukhan <>

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