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Subject: Re: [PATCH 0/2] Fix issue with not starting nesting guests on my
 system
To:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org
Cc:     "H. Peter Anvin" <hpa@zytor.com>, Tao Xu <tao3.xu@intel.com>,
        Sean Christopherson <sean.j.christopherson@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>
References: <20200523161455.3940-1-mlevitsk@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <c12376a8-fc98-5785-e4b6-5c682afd3cd6@redhat.com>
Date:   Wed, 27 May 2020 19:00:13 +0200
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On 23/05/20 18:14, 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(+)
> 

Queued for 5.7, thanks (with cosmetic touches to the commit message, and
moving the "case" earlier to avoid conflicts).

Paolo