Re: [PATCH v8 4/4] kvm: vmx: virtualize split lock detection

[Xiaoyao Li]

On 4/16/2020 5:22 AM, Thomas Gleixner wrote:
> Sean,
>
> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
> > On Wed, Apr 15, 2020 at 07:43:22PM +0200, Thomas Gleixner wrote:
> > > Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> > > > +static inline void vmx_update_sld(struct kvm_vcpu *vcpu, bool on)
> > > > +{
> > > > +	/*
> > > > +	 * Toggle SLD if the guest wants it enabled but its been disabled for
> > > > +	 * the userspace VMM, and vice versa.  Note, TIF_SLD is true if SLD has
> > > > +	 * been turned off.  Yes, it's a terrible name.
> > >
> > > Instead of writing that useless blurb you could have written a patch
> > > which changes TIF_SLD to TIF_SLD_OFF to make it clear.
> >
> > Hah, that's my comment, though I must admit I didn't fully intend for the
> > editorial at the end to get submitted upstream.
> >
> > Anyways, I _did_ point out that TIF_SLD is a terrible name[1][2], and my
> > feedback got ignored/overlooked.  I'd be more than happy to write a
> > patch, I didn't do so because I assumed that people wanted TIF_SLD as the name for
> > whatever reason.
>
> I somehow missed that in the maze of mails regarding this stuff. I've
> already written a patch to rename it to TIF_SLD_DISABLED which is pretty
> self explaining. But see below.
[...]
> > > > @@ -1188,6 +1217,10 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
> > > >   #endif
> > > >
> > > > 	vmx_set_host_fs_gs(host_state, fs_sel, gs_sel, fs_base, gs_base);
> > > > +
> > > > +	vmx->host_sld_on = !test_thread_flag(TIF_SLD);
> > >
> > > This inverted storage is non-intuitive. What's wrong with simply
> > > reflecting the TIF_SLD state?
> >
> > So that the guest/host tracking use the same polairy, and IMO it makes
> > the restoration code more intuitive, e.g.:
> >
> > 	vmx_update_sld(&vmx->vcpu, vmx->host_sld_on);
> > vs
> > 	vmx_update_sld(&vmx->vcpu, !vmx->host_tif_sld);
> >
> > I.e. the inversion needs to happen somewhere.
>
> Correct, but we can make it consistently use the 'disabled' convention.
>
> I briefly thought about renaming the flag to TIF_SLD_ENABLED, set it by
> default and update the 5 places where it is used. But that's
> inconsistent as well simply because it does not make any sense to set
> that flag when detection is not available or disabled on the command
> line.

Assuming you'll pick TIF_SLD_DISABLED, I guess we need to set this flag 
by default for the case SLD is no available or disabled on the command, 
for consistency?