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

[Xiaoyao Li]

On 3/26/2020 7:08 PM, Thomas Gleixner wrote:
> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> 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.