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 14:31:28 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xiaoyao Li <xiaoyao.li@...el.com>, 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

Xiaoyao Li <xiaoyao.li@...el.com> writes:
> 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.

Then please spell it out in the changelog. Changelogs which need crystalballs
to decode are pretty useless.

Thanks,

        tglx