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Date:   Tue, 26 May 2020 18:22:21 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Krish Sadhukhan <krish.sadhukhan@oracle.com>
Cc:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Tao Xu <tao3.xu@intel.com>,
        Jim Mattson <jmattson@google.com>,
        linux-kernel@vger.kernel.org, Joerg Roedel <joro@8bytes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Wanpeng Li <wanpengli@tencent.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Jingqi Liu <jingqi.liu@intel.com>
Subject: Re: [PATCH 0/2] Fix issue with not starting nesting guests on my
 system
Message-ID: <20200527012221.GE31696@linux.intel.com>
References: <20200523161455.3940-1-mlevitsk@redhat.com>
 <d80609d2-ca7b-feea-cb5e-6e45d2116eae@oracle.com>
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On Tue, May 26, 2020 at 06:03:32PM -0700, Krish Sadhukhan wrote:
> 
> 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.

It is unless you want to add a fallthrough annotation.