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Subject: Re: [PATCH] KVM: VMX: Enable Notify VM exit
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Andy Lutomirski <luto@amacapital.net>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Xiaoyao Li <xiaoyao.li@intel.com>
References: <20201102061445.191638-1-tao3.xu@intel.com>
 <CALCETrVqdq4zw=Dcd6dZzSmUZTMXHP50d=SRSaY2AV5sauUzOw@mail.gmail.com>
 <20201102173130.GC21563@linux.intel.com>
From:   Tao Xu <tao3.xu@intel.com>
Message-ID: <34576238-eedf-4a94-880a-c961d2d5b237@intel.com>
Date:   Tue, 3 Nov 2020 13:35:08 +0800
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On 11/3/20 1:31 AM, Sean Christopherson wrote:
> On Mon, Nov 02, 2020 at 08:43:30AM -0800, Andy Lutomirski wrote:
>> On Sun, Nov 1, 2020 at 10:14 PM Tao Xu <tao3.xu@intel.com> wrote:
>>> 2. Another patch to disable interception of #DB and #AC when notify
>>> VM-Exiting is enabled.
>>
>> Whoa there.
>>
>> A VM control that says "hey, CPU, if you messed up and livelocked for
>> a long time, please break out of the loop" is not a substitute for
>> fixing the livelocks.  So I don't think you get do disable
>> interception of #DB and #AC.
> 
> I think that can be incorporated into a module param, i.e. let the platform
> owner decide which tool(s) they want to use to mitigate the legacy architecture
> flaws.
> 
>> I also think you should print a loud warning
> 
> I'm not so sure on this one, e.g. userspace could just spin up a new instance
> if its malicious guest and spam the kernel log.
> 
>> and have some intelligent handling when this new exit triggers.
> 
> We discussed something similar in the context of the new bus lock VM-Exit.  I
> don't know that it makes sense to try and add intelligence into the kernel.
> In many use cases, e.g. clouds, the userspace VMM is trusted (inasmuch as
> userspace can be trusted), while the guest is completely untrusted.  Reporting
> the error to userspace and letting the userspace stack take action is likely
> preferable to doing something fancy in the kernel.
> 
> 
> Tao, this patch should probably be tagged RFC, at least until we can experiment
> with the threshold on real silicon.  KVM and kernel behavior may depend on the
> accuracy of detecting actual attacks, e.g. if we can set a threshold that has
> zero false negatives and near-zero false postives, then it probably makes sense
> to be more assertive in how such VM-Exits are reported and logged.
> 
Sorry, I should add RFC tag for this patch. I will add it next time.