linux-kernel - Re: [PATCH v6 8/8] kvm: vmx: virtualize split lock detection

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Date:   Thu, 26 Mar 2020 20:31:02 +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>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        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>
Subject: Re: [PATCH v6 8/8] kvm: vmx: virtualize split lock detection

On 3/26/2020 7:08 PM, Thomas Gleixner wrote:
> Xiaoyao Li <xiaoyao.li@...el.com> writes:
>> On 3/25/2020 9:41 AM, Thomas Gleixner wrote:
>>> If you really want to address that scenario, then why are you needing
>>> any of those completely backwards interfaces at all?
>>>
>>> Just because your KVM exception trap uses the host handling function
>>> which sets TIF_SLD?
>>>    
>> Yes. just because KVM use the host handling function.
> 
>> If you disallow me to touch codes out of kvm. It can be achieved with
> 
> Who said you cannot touch code outside of KVM?
> 
>> Obviously re-use TIF_SLD flag to automatically switch MSR_TEST_CTRL.SLD
>> bit when switch to/from vcpu thread is better.
> 
> What's better about that?
> 
> TIF_SLD has very well defined semantics. It's used to denote that the
> SLD bit needs to be cleared for the task when its scheduled in.
> 
> So now you overload it by clearing it magically and claim that this is
> better.
> 
> vCPU-thread
> 
>     user space (qemu)
>       triggers #AC
>         -> exception
>             set TIF_SLD
> 
>     iotctl()
>       vcpu_run()
>         -> clear TIF_SLD
> 
> It's not better, it's simply wrong and inconsistent.
> 
>> And to virtualize SLD feature as full as possible for guest, we have to
>> implement the backwards interface. If you really don't want that
>> interface, we have to write code directly in kvm to modify TIF_SLD flag
>> and MSR_TEST_CTRL.SLD bit.
> 
> Wrong again. KVM has absolutely no business in fiddling with TIF_SLD and
> the function to flip the SLD bit is simply sld_update_msr(bool on) which
> does not need any KVMism at all.
> 
> There are two options to handle SLD for KVM:
> 
>     1) Follow strictly the host rules
> 
>        If user space or guest triggered #AC then TIF_SLD is set and that
>        task is excluded from ever setting SLD again.

Obviously, it's not about to virtualize SLD for guest. So we don't pick 
this one.

>     2) Track KVM guest state separately
> 
>        vcpu_run()
>          if (current_has(TIF_SLD) && guest_sld_on())
>            sld_update_msr(true);
>          else if (!current_has(TIF_SLD) && !guest_sld_on())
>            sld_update_msr(false);
>          vmenter()
>            ....
>          vmexit()
>          if (current_has(TIF_SLD) && guest_sld_on())
>            sld_update_msr(false);
>          else if (!current_has(TIF_SLD) && !guest_sld_on())
>            sld_update_msr(true);
> 
>        If the guest triggers #AC then this solely affects guest state
>        and does not fiddle with TIF_SLD.
> 

OK. So when host is sld_warn, guest's SLD value can be loaded into 
hardware MSR when vmenter.

I'll go with this option.