linux-kernel - Re: [PATCH] KVM: VMX: Enable Notify VM exit

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Date:   Mon, 2 Nov 2020 19:25:37 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Tao Xu <tao3.xu@...el.com>, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH] KVM: VMX: Enable Notify VM exit

On 02/11/20 19:01, Andy Lutomirski wrote:
> What's the point?  Surely the kernel should reliably mitigate the
> flaw, and the kernel should decide how to do so.

There is some slowdown in trapping #DB and #AC unconditionally.  Though
for these two cases nobody should care so I agree with keeping the code
simple and keeping the workaround.

Also, why would this trigger after more than a few hundred cycles,
something like the length of the longest microcode loop?  HZ*10 seems
like a very generous estimate already.

Paolo

>>> 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.
> pr_warn_once()?  If this triggers, it's a *bug*, right?  Kernel or CPU.
>