linux-kernel - Re: [PATCH v5 1/9] x86/split_lock: Rework the initialization flow of split lock detection

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Date:   Tue, 24 Mar 2020 19:51:19 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        hpa@...or.com, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v5 1/9] x86/split_lock: Rework the initialization flow of
 split lock detection

On 3/24/2020 1:02 AM, Thomas Gleixner wrote:
> Xiaoyao Li <xiaoyao.li@...el.com> writes:
> 
>> Current initialization flow of split lock detection has following issues:
>> 1. It assumes the initial value of MSR_TEST_CTRL.SPLIT_LOCK_DETECT to be
>>     zero. However, it's possible that BIOS/firmware has set it.
> 
> Ok.
> 
>> 2. X86_FEATURE_SPLIT_LOCK_DETECT flag is unconditionally set even if
>>     there is a virtualization flaw that FMS indicates the existence while
>>     it's actually not supported.
>>
>> 3. Because of #2, KVM cannot rely on X86_FEATURE_SPLIT_LOCK_DETECT flag
>>     to check verify if feature does exist, so cannot expose it to
>>     guest.
> 
> Sorry this does not make anny sense. KVM is the hypervisor, so it better
> can rely on the detect flag. Unless you talk about nested virt and a
> broken L1 hypervisor.
> 

Yeah. It is for the nested virt on a broken L1 hypervisor.