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

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Message-ID: <06dbea28-4b4a-4f73-2ad2-9b76a8ca4704@intel.com>
Date:   Tue, 3 Nov 2020 14:39:30 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        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>
Subject: Re: [PATCH] KVM: VMX: Enable Notify VM exit

On 11/3/2020 2:25 AM, Paolo Bonzini wrote:
> 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.

OK.

> 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.
> 

As Sean said in another mail, 1/10 tick should be a placeholder.
Glad to see all of you think it should be smaller. We'll come up with 
more reasonable candidate once we can test on real silicon.